intel_adsp: common: gdbstub: Correct z_gdb_backend_init() return type

The z_gdb_backend_init() is defined to return with success/error code
with return type of int.

Fixes: a9c47a47a4f3 ("intel_adsp: cavs: add gdb support")
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>