The 60xx chips, and possibly others, incorrectly assert DEV_IRQ interrupts
on a regular basis.  The cause of this is under investigation (by me and
in theory by Marvell also), but regardless we do need to deal with these events.

This patch tidies up some interrupt handler code, and ensures that we ignore
DEV_IRQ interrupts when the drive still  has ATA_BUSY asserted.
Signed-off-by: Mark Lord <liml@rtr.ca>
Signed-off-by: Jeff Garzik <jeff@garzik.org>