use minimal unmasked rate for multicast frames.

use minimal unmasked rate for multicast frames.

Current FW has ratemask interface and use it to set
minmal unmasked rate for beacons and mgmt frames.

This patch will allow to do the same with mcast.

Fallowing command will force mcast frames to be send with
54Mbit:
iw wlan0 set bitrates legacy-2.4 54

If multiple rate are set, then minimal rate will be used,
for example in this case 6Mbit:
iw wlan0 set bitrates legacy-2.4 6 54

This patch also should affect rate of injected frames.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>