Add @is_cmd to ata_tf_to_fis(). This controls bit 7 of the second
byte which tells the device whether this H2D FIS is for a command or
not. This cleans up ahci a bit and will be used by PMP.
Signed-off-by: Tejun Heo <htejun@gmail.com> Signed-off-by: Jeff Garzik <jeff@garzik.org>