mac80211: allow hardware scan to fall back to software

In some cases, like in the rsi driver hardware scan offload, there
may be scenarios in which hardware scan might not be available or
desirable.

Allow drivers to cope with this by letting them fall back to software
scan by returning the special value 1 from the hardware scan method.

Requested-by: Sushant Kumar Mishra <sushant2k1513@gmail.com>
Requested-by: Siva Rebbagondla <siva.rebbagondla@redpinesignals.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>