nl80211/cfg80211: add max_sched_scan_ssids in the hw description

Some chips can scan more SSIDs with a single scheduled scan command
than with a single normal scan command (eg. wl12xx chips).

To support this, this patch creates a separate hardware description
element that describes the amount of SSIDs supported in scheduled
scans.

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