wifi: nl80211: remove redundant err variable

Return value from rdev_set_mcast_rate() directly instead of
taking this in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jinpeng Cui <cui.jinpeng2@zte.com.cn>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>