target/loongarch: fix -Werror=maybe-uninitialized false-positive

target/loongarch: fix -Werror=maybe-uninitialized false-positive

../target/loongarch/gdbstub.c:55:20: error: ‘val’ may be used uninitialized [-Werror=maybe-uninitialized]
   55 |             return gdb_get_reg32(mem_buf, val);
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
../target/loongarch/gdbstub.c:39:18: note: ‘val’ was declared here
   39 |         uint64_t val;

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>