cfg80211: refactor nl80211_start_sched_scan so it can be reused

For net detect, we will need to reuse most of the scheduled scan
parsing function, but not all, so split out the attributes parsing
part out of the main start sched_scan function.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>