mac80211: add support for HW scheduled scan

Implement support for HW scheduled scan.  The mac80211 code doesn't perform
scheduled scans itself, but calls the driver to start and stop scheduled
scans.

This patch also creates a trace event class to be used by drv_hw_scan
and the new drv_sched_scan_start and drv_sched_stop functions, in
order to avoid duplicate code.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>