MIPS: Lantiq: Fix check for return value of request_mem_region()

request_mem_region() returns a pointer and not an integer with an error
value. A check for "< 0" on a pointer will cause problems, replace it
with not null checks instead. This was found with sparse.

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@lantiq.com>
Acked-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11395/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>