SATA: use NULL for ptrs

Fix sparse warnings in SATA:
drivers/ata/sata_sil.c:342:9: warning: Using plain integer as NULL pointer
drivers/ata/sata_mv.c:2056:55: warning: Using plain integer as NULL pointer

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>