sata_qstor workaround for spurious interrupts.

The qstor hardware generates spurious interrupts from time to time when
switching in and out of packet mode.  These eventually result in the
IRQ being disabled, which kills other devices sharing this IRQ with us.

This workaround isn't perfect, but it's about the best we can do for
this hardware.  Spurious interrupts will still happen, but won't be
logged as such, and therefore won't cause the IRQ to be inadvertently
disabled.
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>