libata: force PIO on IOMEGA ZIP 250 ATAPI

IOMEGA ZIP 250 ATAPI claims MWDMA0 support but fails SETXFERMODE if
asked to configure itself to MWDMA0.  Force PIO.

This fixes bugzilla bug#8497.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Calvin Walton <calvin.walton@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>