libata: reorganize ata_eh_reset() no reset method path

Reorganize ata_eh_reset() such that @prereset() is called even when no
reset method is available and if block is used instead of goto to skip
actual reset.  This makes no reset case behave better (readiness wait)
and future changes easier.

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