libata: move ata_set_mode() to libata-eh.c

Move ata_set_mode() to libata-eh.c.  ata_set_mode() is surely an EH
action and will be more tightly coupled with the rest of error
handling.  Move it to libata-eh.c.

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