[libata] sata_mv: minor bug fixes, enhancements, and cleanups (prep for new EH)

* Continue replacing "CONSTANT & var" tests with "var & CONSTANT"
* Don't clear EDMA_CFG_NCQ_GO_ON_ERR on Gen-IIE, where that bit does
  not exist
* Set I/O Id field in descriptor, where present.  Appears to work
  fine on all versions, even though queueing is still disabled.
* call pci_set_mwi(), to (a) make sure cacheline size is set properly,
  and (b) enable MWI transactions
* Remove never-used handling of coalescing interrupt bits (these events
  are always masked)

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