iwlagn: support off-channel TX

Add support to iwlagn for off-channel TX. The
microcode API for this is a bit strange in that
it uses a hacked-up scan command, so the scan
code needs to change quite a bit to accomodate
that and be able to send it out.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>