wl12xx: add band field to wlvif

add band field into the per-interface data.

mac80211 configures some values (e.g. band, channel)
globally, while we configure them per-interface.

In order to make it easier to keep track of the
configured value for each value while keeping sync
with mac80211, save these values both globally
and per-vif.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>