wl12xx: add IEEE80211_HW_SPECTRUM_MGMT bit to the hw flags

Set the spectrum management bit in the hw flags so that mac80211 will
set the WLAN_CAPABILITY_SPECTRUM_MGMT bit in association requests
(which in practice means that we support 802.11h spectrum management).

[Reworded the commit log -- Luca.]

Signed-off-by: Shahar Levi <shahar_levi@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>