carl9170: fix allmulticast mode

Currently, the driver only disable multicast filtering when the
FIF_ALLMULTI driver flag has been just set (ie,
if changed_flags& FIF_ALLMULTI and *new_flags& FIF_ALLMULTI) or else
it will reenable multicast filtering.

But next time, this condition will be false and multicast filtering
will be reenabled, even through FIF_ALLMULTI is still set.

This mean that allmulticast only works for less than two minutes in
ad-hoc mode.  This patch fixes that to disable multicast filtering
as long as FIF_ALLMULTI is set.

Signed-off-by: Nicolas Cavallari <cavallar@lri.fr>
Acked-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>