wl12xx: fix identification of beacon packets (debug)

for debugging purposes, wl12xx determines whether a rx packet
is a beacon packet.

however, it checks only the frame_control subtype without checking
the actual packet type, which leads to false identification in some
cases.

use ieee80211_is_beacon instead.

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