mac80211: ignore VHT membership selector when parsing rates

There isn't really much harm in not ignoring, since it doesn't
represent a valid rate, but since we already ignore the HT one
also ignore VHT. Also simplify the code a bit.

Fix a typo in the related comment (pointed out by Arend) while
at it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>