Add a temporary fix to ata_interrupt() to ignore weird interrupts seen on

Add a temporary fix to ata_interrupt() to ignore weird interrupts seen on
SMP+ATAPI+DMA. For some reason, the ATAPI device asserts INTRQ while it's
otherwise completely ready to fulfill the request. DRQ is set in the status
register, and no matter how I read the specs, this just should never
happen. Is this actually a race condition? I have no idea :(. Ignoring the
interrupt, however, does not seem to produce adverse results (tested with
a DragonFlyBSD livecd with SMP kernel).