sata_mv: fix SoC interrupt breakage

For some reason, sata_mv doesn't clear interrupt status during init
when it's running on an SoC host adapter.  If the bootloader has
touched the SATA controller before starting Linux, Linux can end up
enabling the SATA interrupt with events pending, which will cause the
interrupt to be marked as spurious and then be disabled, which then
breaks all further accesses to the controller.

This patch makes the SoC path clear interrupt status on init like in
the non-SoC case.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>