wl12xx: avoid redundant join on interface reconfiguration

ieee80211_reconfig() sets most of the "changed" flags regardless
of the actual change (e.g. BSS_CHANGED_ASSOC will be set even if
the interface is still not associated). in this case the driver
will issue some unneeded commands.

Since the driver relies solely on the BSS_CHANGED_ASSOC flag,
without checking if there was an actual change, it will end up
issuing unjoin() and dummy_join() commands, although it was
never associated and should just remain idle.

Avoid it by checking the actual state change, in addition to the
"changed" flag.

(there seem to be more redundant configuration commands being
issued, but they shouldn't harm)

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