mac80211: move setting WIPHY_FLAG_SUPPORTS_SCHED_SCAN into drivers

mac80211 currently sets WIPHY_FLAG_SUPPORTS_SCHED_SCAN based on whether
the start_sched_scan operation is supported or not, but that will not
be correct for all drivers, we're adding scheduled scan to the iwlmvm
driver but it depends on firmware support.

Therefore, move setting WIPHY_FLAG_SUPPORTS_SCHED_SCAN into the drivers
so that they can control it regardless of implementing the operation.

This currently only affects the TI drivers since they're the only ones
implementing scheduled scan (in a mac80211 driver.)

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