libata: improve ata_std_prereset()

This patch updates ata_std_prereset() as follows.

* Don't fail on phy resume failure.  Just whine and continue.  Failure
  from prereset makes libata abort whole reset sequence and give up
  the port, so prereset() should be best effort.  This is more
  important with the coming EH updates as prereset() will be called
  with shorter timeout.

* If ata_wait_ready() fails, whine and request hardreset instead.

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