ata: pata_macio: fix comparing pointer to 0

Fixes coccicheck warning:

./drivers/ata/pata_macio.c:982:31-32:
	WARNING comparing pointer to 0, suggest !E

Compare pointer-typed values to NULL rather than 0.

Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>