pata_it821x: fix lost interrupt with atapi devices

Fix "lost" interrupt problem when using dma with CD/DVD drives in some
configurations.  This problem can make installing linux from media
impossible for distro's that have switched to libata-only configurations.

The simple fix is to eliminate the use of dma for reading drive status, etc,
by checking the number of bytes to transferred.

This change will only affect the behavior of atapi devices, not disks.
There is more info at http://bugzilla.redhat.com/show_bug.cgi?id=242229
This patch is for 2.6.22.1

Signed-off-by: Jeff Norden <jnorden@math.tntech.edu>
Reviewed-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>