gdbhio: fix _gdbHioExportFileMode, closes #604

[silentium83]

Fixes the following libctru compile error by returning the already computed local variable gdbMode instead of the unchanged function parameter mode:

/home/silentium/projects/libctru/libctru/source/gdbhio.c: In function '_gdbHioExportFileMode':
/home/silentium/projects/libctru/libctru/source/gdbhio.c:176:23: error: variable 'gdbMode' set but not used [-Werror=unused-but-set-variable=]
  176 |         gdbhio_mode_t gdbMode = 0;
      |                       ^~~~~~~
cc1: all warnings being treated as errors
make[1]: *** [/opt/devkitpro/devkitARM/base_rules:40: gdbhio.o] Error 1
make: *** [Makefile:116: lib/libctru.a] Error 2