configure: check for $download value properly

If configure was invoked with --disable-download and git
submodules were not checked out a warning is produced and the
configure script fails. But the $download variable (which
reflects the enable/disable download argument) is checked for in
a weird fashion:

  test -f "$download" = disabled

Drop the '-f' to check for the actual value of the variable.

Fixes: 2019cab ("meson: subprojects: replace submodules with wrap files", 2023-06-06)
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -767,7 +767,7 @@ if test "$plugins" = "yes" -a "$tcg" = "disabled"; then
 fi
 
 if ! test -f "$source_path/subprojects/keycodemapdb/README" \
-    && test -f "$download" = disabled
+    && test "$download" = disabled
 then
     echo
     echo "ERROR: missing subprojects"