[PATCH] libata: clear only affected flags during ata_dev_configure()

ata_dev_configure() should not clear dynamic device flags determined
elsewhere.  Lower eight bits are reserved for feature flags, define
ATA_DFLAG_CFG_MASK and clear only those bits before configuring
device.  Without this patch, ATA_DFLAG_PIO gets turned off during
revalidation making PIO mode unuseable.

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