[PATCH] libata: add per-dev pio/mwdma/udma_mask

Add per-dev pio/mwdma/udma_mask.  All transfer mode limits used to be
applied to ap->*_mask which unnecessarily restricted other devices
sharing the port.  This change will also benefit later EH speed down
and hotplug.

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