sparc32: fix sparse warnings in ioport.c

Fix following warnings:
ioport.c:189:38: warning: Using plain integer as NULL pointer
ioport.c:78:25: warning: symbol 'sparc_iomap' was not declared. Should it be static?
ioport.c:403:20: warning: symbol 'sbus_dma_ops' was not declared. Should it be static?
ioport.c:684:39: warning: Using plain integer as NULL pointer

Add one missing prototype, and use NULL.
sbus_dma_ops declared static.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>