sata_via: don't diddle with ATA_NIEN in ->freeze

vt6420 completely loses its ability to raise IRQ for ATAPI devices if
ATA_NIEN is diddled with in ->freeze.  Further investigation is
necessary to determine whether this problem is shared on other
controllers but it doesn't seem to be at this point.

Make vt6420's ->freeze only clear IRQ to fix this problem.  This makes
vt6420 relatively more prone to IRQ storms but the controller is way
too braindamaged to worry about that anyway.

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