libata: hardreset on SERR_INTERNAL

There was a rare report where SB600 reported SERR_INTERNAL and SRST
couldn't get it out of the failure mode.  Hardreset on SERR_INTERNAL.
As the problem is intermittent, whether this fixes the problem or not
hasn't been verified yet, but hardresetting the channel on internal
error is a good idea anyway.

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