ide: Clear DRQ after handling all expected accesses

ide: Clear DRQ after handling all expected accesses

This is additional hardening against an end_transfer_func that fails to
clear the DRQ status bit. The bit must be unset as soon as the PIO
transfer has completed, so it's better to do this in a central place
instead of duplicating the code in all commands (and forgetting it in
some).

[ This is XSA-138 / CVE-2015-5154. ]

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 7f057440b31da38196e3398fd1b618fc36ad97d6)