ahci: Fix crashes on duplicate BH registration

If ahci_dma_set_inactive is called a while there is still a pending BH
from a previous run, we will crash on the second run of
ahci_check_cmd_bh as it overwrites AHCIDevice::check_bh. Avoid this
broken and redundant duplicate registration.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
