sata_promise: fix irq clearing buglets

This patch fixes two bugs in sata_promise's irq status clearing paths:
1. When clearing the irq status for a specific port, the driver
   read the global SEQMASK register. This is wrong because that
   clears the irq status for _all_ ports.
2. pdc_thaw() incorrectly added the PDC_INT_SEQMASK host register
   offset to a per-port ata engine base address. This resulted in
   it reading the unrelated PDC_PKT_SUBMIT register, which did not
   have the desired irq status clearing effect.

In both cases the fix is to read from the port's Command/Status
register. This also matches what Promise's own driver does.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>