ahci: Force ICC bits in PxCMD to zero

The AHCI spec requires that the HBA sets the ICC bits to zero after the
ICC change is done. Since we don't do any ICC change, force the bits to
zero all the time.

This fixes delays with some OSs (e.g. OpenBSD) waiting for the ICC bits
to change to 0.

Signed-off-by: Stefan Fritsch <sf@sfritsch.de>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: E1ZFpg7-00027N-HW@eru.sfritsch.de
Signed-off-by: John Snow <jsnow@redhat.com>