ata: sata_dwc_460ex: tidy up sata_dwc_clear_dmacr()

This consolidates the reads from each of the if/else branches
to one place making the code a lot nicer to look at.

Tested-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Tejun Heo <tj@kernel.org>