MIPS: Lantiq: Fix another request_mem_region() return code check

MIPS: Lantiq: Fix another request_mem_region() return code check

Hauke already fixed a couple of them, but one instance remains
that checks for a negative integer when it should check
for a NULL pointer:

arch/mips/lantiq/xway/sysctrl.c: In function 'ltq_soc_init':
arch/mips/lantiq/xway/sysctrl.c:473:19: error: ordered comparison of pointer with integer zero [-Werror=extra]

Fixes: 6e807852676a ("MIPS: Lantiq: Fix check for return value of request_mem_region()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15043/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>