mac80211: minstrel_ht: remove unused variable 'mg' in minstrel_ht_next_jump_rate()

mac80211: minstrel_ht: remove unused variable 'mg' in minstrel_ht_next_jump_rate()

GCC reports the following warning with W=1:

net/mac80211/rc80211_minstrel_ht.c:871:34: warning:
 variable 'mg' set but not used [-Wunused-but-set-variable]
  871 |  struct minstrel_mcs_group_data *mg;
      |                                  ^~

This variable is not used in function , this commit
remove it to fix the warning.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210326024843.987941-1-weiyongjun1@huawei.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>