ata: pata_pxa: Fix potential NULL pointer dereference in pxa_ata_probe()

devm_ioremap() returns NULL on error. Currently, pxa_ata_probe() does
not check for this case, which can result in a NULL pointer dereference.

Add NULL check after devm_ioremap() to prevent this issue.

Fixes: 2dc6c6f1 ("[ARM] pata_pxa: DMA-capable PATA driver")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>