[PATCH] libata: pass qc around intead of ap during PIO

The current code passes pointer to ap around and repeatedly performs
ata_qc_from_tag() to access the ongoing qc.  This is unnatural and
makes EH synchronization cumbersome.  Make PIO codes deal with qc
instead of ap.

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