Update ata_exec_internal() such that it uses new EH framework.
->post_internal_cmd() is always invoked regardless of completion
status.  Also, when ata_exec_internal() detects a timeout condition
and new EH is in place, it freezes the port as timeout for normal
commands would do.

Note that ata_port_flush_task() is called regardless of
wait_for_completion status.  This is necessary as exceptions unrelated
to the qc can abort the qc, in which case PIO task could still be
running after the wait for completion returns.
Signed-off-by: NTejun Heo <htejun@gmail.com>