microblaze: Fix sparse warnings - signal.c

Warning log:
CHECK   arch/microblaze/kernel/signal.c
arch/microblaze/kernel/signal.c:200:9: warning: Using plain integer as NULL pointer
arch/microblaze/kernel/signal.c:201:9: warning: incorrect type in initializer (different address spaces)
arch/microblaze/kernel/signal.c:201:9:    expected void [noderef] *volatile __gu_val<asn:1>
arch/microblaze/kernel/signal.c:201:9:    got void *<noident>

Signed-off-by: Michal Simek <monstr@monstr.eu>