[PATCH] libata: implement ata_wait_register()

As waiting for some register bits to change seems to be a common
operation shared by some controllers, implement helper function
ata_wait_register().  This function also takes care of register write
flushing.

Note that the condition is inverted, the wait is over when the masked
value does NOT match @val.  As we're waiting for bits to change, this
test is more powerful and allows the function to be used in more
places.

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