ssb: fix sparse warnings in driver_chipcommon_sflash.c

  CHECK   drivers/ssb/driver_chipcommon_sflash.c
drivers/ssb/driver_chipcommon_sflash.c:40:11: warning: Using plain integer as NULL pointer
drivers/ssb/driver_chipcommon_sflash.c:58:11: warning: Using plain integer as NULL pointer
drivers/ssb/driver_chipcommon_sflash.c:69:11: warning: Using plain integer as NULL pointer

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>