isobusfs: fix clang warnings

[olerem]

Fix following clang warnings:

  CC       isobusfs/isobusfs_srv_fa.o
  isobusfs/isobusfs_srv_fa.c:387:6: warning: variable 'handle' is used
  uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
        if (ret < 0) {
            ^~~~~~~
  isobusfs/isobusfs_srv_fa.c:411:15: note: uninitialized use occurs here
        res.handle = handle;
                     ^~~~~~
  isobusfs/isobusfs_srv_fa.c:387:2: note: remove the 'if' if its condition is
  always false
        if (ret < 0) {
        ^~~~~~~~~~~~~~
  isobusfs/isobusfs_srv_fa.c:368:6: warning: variable 'handle' is used
  uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
        if (name_len > ISOBUSFS_MAX_PATH_NAME_LENGTH) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  isobusfs/isobusfs_srv_fa.c:411:15: note: uninitialized use occurs here
        res.handle = handle;
                     ^~~~~~
  isobusfs/isobusfs_srv_fa.c:368:2: note: remove the 'if' if its condition is
  always false
        if (name_len > ISOBUSFS_MAX_PATH_NAME_LENGTH) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  isobusfs/isobusfs_srv_fa.c:362:6: warning: variable 'handle' is used
  uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
        if (name_len > msg->len - sizeof(*req)) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  isobusfs/isobusfs_srv_fa.c:411:15: note: uninitialized use occurs here
        res.handle = handle;
                     ^~~~~~
  isobusfs/isobusfs_srv_fa.c:362:2: note: remove the 'if' if its condition is
  always false
        if (name_len > msg->len - sizeof(*req)) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  isobusfs/isobusfs_srv_fa.c:356:6: warning: variable 'handle' is used
  uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
        if (!client) {
            ^~~~~~~
  isobusfs/isobusfs_srv_fa.c:411:15: note: uninitialized use occurs here
        res.handle = handle;
                     ^~~~~~
  isobusfs/isobusfs_srv_fa.c:356:2: note: remove the 'if' if its condition is
  always false
        if (!client) {
        ^~~~~~~~~~~~~~
  isobusfs/isobusfs_srv_fa.c:352:16: note: initialize the variable 'handle' to
  silence this warning
        uint8_t handle;
                      ^
                       = '\0'
  isobusfs/isobusfs_srv_fa.c:505:9: warning: variable 'entry_count' set but not
  used [-Wunused-but-set-variable]
        size_t entry_count = 0;
               ^
  5 warnings generated.

[marckleinebudde]

closes #507