libata: kill dead code paths in reset path

Some code paths which had been made obsolete by recent reset
simplification were still around.  Kill them.

* ata_eh_reset() checked for ATA_DEV_UNKNOWN to determine
  classification failure.  This is no longer applicable.

* ata_do_reset() should convert ATA_DEV_UNKNOWN to ATA_DEV_NONE
  regardless of reset result (e.g. -EAGAIN).

* LLDs don't need to convert ATA_DEV_UNKNOWN to ATA_DEV_NONE.

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