[PATCH] libata: add FIXME above ata_dev_xfermask()

Add FIXME above ata_dev_xfermask noting that the current
implementation limits all transfer modes to the fastest of the slowest
device on a port which isn't necessary.

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