libata: factor out ata_eh_schedule_probe()

Factor out ata_eh_schedule_probe() from ata_eh_handle_dev_fail() and
ata_eh_recover().  This is to improve maintainability and make future
changes easier.

In the previous revision, ata_dev_enabled() test was accidentally
dropped while factoring out.  This problem was spotted by Bartlomiej.

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