mac80211: fix the setting of extended supported rate IE

The patch "mac80211: select and adjust bitrates according to
channel mode" causes regression and breaks the extended supported rate
IE setting. Since "i" is starting with 8, so this is not necessary
to introduce "skip" here.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@cozybit.com>
Signed-off-by: Colleen Twitty <colleen@cozybit.com>
Reviewed-by: Jason Abele <jason@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>