staging: wfx: drop multicast filtering

staging: wfx: drop multicast filtering

The device allows to filter multicast frames. The driver has the
necessary code to take advantage of this feature. However, some bugs
has been reported on this feature. So, it was temporary disabled.

Finally, the things work well as-is for more than one year now. So there
is no plan to enable this feature in near future.

Since we dislike to maintain dead code, drop it.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200907101521.66082-7-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>