libata: fix reporting of drained bytes when clearing DRQ

When we drain data from a device to clear DRQ during error recovery,
the number of bytes reported as drained is too low by a factor of 2
because the count is actually reporting the number of words drained,
not bytes. Fix this.

Signed-off-by: Robert Hancock <hancockrwd@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>