[libata] sata_nv: s/spin_lock_irqsave/spin_lock/ in irq handler

We don't need to use the heavier spin lock in the irq handler.

It's quite possible we can do this in nv_generic_interrupt() as well,
but I didn't take the time to pursue that train of thought.

Signed-off-by: Jeff Garzik <jeff@garzik.org>