iwlagn: map command buffers BIDI

Evidently, the device sometimes wants to write back
to command buffers, even if I see no reason why it
should. Allow it to do that.

Tested-by: Andy Lutomirski <luto@mit.edu>
Tested-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>