pata_it821x: use raw nbytes in check_atapi_dma

pata_it821x needs to look at raw request size in check_atapi_dma().

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Jeff Garzik <jeff@garzik.org>