iw: move generic sched scan parsing code out of net detect

iw: move generic sched scan parsing code out of net detect

The scheduled scan structure is pretty much the same as the net-detect
WoWLAN trigger's.  Move the bulk of the command line parsing code to
a generic function so we can reuse it for sched_scan.

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