sata_mv: cosmetic preparations for IRQ coalescing

Various cosmetic changes in preparation for the IRQ coalescing feature.

Note that the various MV_IRQ_COAL_* definitions are restored/renamed
in the folloup patch which adds IRQ coalescing to the driver.

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