libata: make EH fail gracefully if no reset method is available

When no reset method is available, libata currently oopses.  Although
the condition can't happen unless there's a bug in a low level driver,
oopsing isn't the best way to report the error condition.  Complain,
dump stack and fail reset instead.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>