MIPS: ralink: Fix request_mem_region error handling

MIPS: ralink: Fix request_mem_region error handling

request_mem_region returns a NULL pointer on error, comparing it
against a number results in a warning:

arch/mips/ralink/of.c: In function 'plat_of_remap_node':
arch/mips/ralink/of.c:45:15: error: ordered comparison of pointer with integer zero [-Werror=extra]
arch/mips/ralink/irq.c: In function 'intc_of_init':
arch/mips/ralink/irq.c:167:15: error: ordered comparison of pointer with integer zero [-Werror=extra]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: John Crispin <john@phrozen.org>
Cc: Tobias Wolf <dev-NTEO@vplace.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15045/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>