[libata] sata_mv: Micro-optimization and cleanups

* Micro-optimization in the EDMA interrupt handling code
* s/EDMA_ERR_CRBQ_PAR/EDMA_ERR_CRQB_PAR/
* Document EDMA Error Interrupt Cause register bits

Signed-off-by: Jeff Garzik <jeff@garzik.org>