libata: allow LLDs w/o any reset method

Some old SFF controllers don't have any way to reset the channel.
Currently, this isn't supported and libata EH causes an oops.  Allow
LLDs w/o any reset method and just assume ATA class in such cases.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jeff Garzik <jeff@garzik.org>