[PATCH] libata: add ap->pflags and move core dynamic flags to it

ap->flags is way too clamped.  Separate out core dynamic flags to
ap->pflags.  ATA_FLAG_DISABLED is a dynamic flag but left alone as
it's referenced by a lot of LLDs and it's gonna be removed once all
LLDs are converted to new EH.

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