AHCI: Do not (re)map FB/CLB buffers while not running

The FIS Receive Buffer and Command List Buffer pointers
should not be edited while the FIS receive engine or
Command Receive engines are running.

Currently, we attempt to re-map the buffers every time they
are adjusted, but while the AHCI engines are off, these registers
may contain stale values, so we should not attempt to re-map these
values until the engines are reactivated.

Reported-by: Jordan Hargrave <jharg93@gmail.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1426283454-15590-2-git-send-email-jsnow@redhat.com