pata_optidma: fix checking of DMA state

Checking if DMA is enabled should be done via the
ata_dma_enabled helper function, since the init state
0xff indicates disabled.
Change based on code review, not tested due to lack of hardware.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>