add stats for ata busy interrupts and those associated with no drive (spurious interrupts ?).
also added a fix derrived from 9atom code (sdide.c) which sends an acknowledge to the interface
in the spurious case and reduced the inmber of interrupts/sec by an order of magnitude. 

here is my /dev/sdctl with the fix applied, sdE and sdF are a
mirror'ed pair on the same IRQ.

	sdE ata port 24F0 ctl 2808 irq 10 busy 563 nildrive 313424 ok 540356
	sdF ata port 24F8 ctl 280C irq 10 busy 283 nildrive 539719 ok 314341

-Steve