Remove unused-parameter errors when LOGGER=swo

[scoudreau]

Describe the PR
Remove errors when compiling with LOGGER=swo.
All arguments of sys_read() must be used in function, otherwise it triggers "unused-parameter" errors.

Additional context
Before this patch, error was:

hw/bsp/board.c: In function '_read':
hw/bsp/board.c:126:44: error: unused parameter 'buf' [-Werror=unused-parameter]
126 | TU_ATTR_USED int sys_read (int fhdl, char *buf, size_t count)
| ~~~~~~^~~
compilation terminated due to -Wfatal-errors.
cc1: all warnings being treated as errors

[hathach]

ah right, thank you for your PR. I was planning to add the SWO read, but then I realize swo is output only :D and kind of forgot to fix this.