MIPS: BCM47XX: fix sparse warnings in board.c

This fixes the following sparse warnings:
arch/mips/bcm47xx/board.c:39:16: warning: Using plain integer as NULL pointer
arch/mips/bcm47xx/board.c:46:16: warning: Using plain integer as NULL pointer
arch/mips/bcm47xx/board.c:53:16: warning: Using plain integer as NULL pointer
arch/mips/bcm47xx/board.c:78:16: warning: Using plain integer as NULL pointer
arch/mips/bcm47xx/board.c:99:16: warning: Using plain integer as NULL pointer
arch/mips/bcm47xx/board.c:109:16: warning: Using plain integer as NULL pointer
arch/mips/bcm47xx/board.c:124:16: warning: Using plain integer as NULL pointer
arch/mips/bcm47xx/board.c:155:16: warning: Using plain integer as NULL pointer
arch/mips/bcm47xx/board.c:177:16: warning: Using plain integer as NULL pointer
arch/mips/bcm47xx/board.c:189:16: warning: Using plain integer as NULL pointer

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6318/