wl12xx: add max_sched_scan_ssids value to the hw description

After commit 5a865ba, we require a separate value to indicate the
number of supported SSIDs in scheduled scans.  This patch adds a
proper value to the wl12xx driver.

This fixes a regression in 3.1-rc3 where scheduled scans were not
working properly with the wl12xx driver.

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