mac80211: support more than one band in scan request

Some drivers (such as iwlmvm) can handle multiple bands in a single
HW scan request. Add a HW flag to indicate that the driver support
this. To hold the required data, create a separate structure for
HW scan request that holds cfg scan request and data about
different parts of the scan IEs.

As this changes the mac80211 API, update all drivers using it to
use the correct new function type/argument.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>