sata_sil: ignore and clear spurious IRQs while executing commands by polling

sata_sil used to trigger HSM error if IRQ occurs during polling
command.  This didn't matter because polling wasn't used in sata_sil.
However, as of 2.6.20, all IDENTIFYs are performed by polling and
device detection sometimes fails due to spurious IRQ.  This patch
makes sata_sil ignore and clear spurious IRQ while executing commands
by polling.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>