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>
Git-commit: 256da02d1806c740be97576a5e8548d658858319
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


CRs-fixed: 1082480
Change-Id: Ib799ce36cc4e39e0db20df978789f57ab9c8eead
Signed-off-by: Srinivas Dasari <dasaris@codeaurora.org>