cfg80211: refactor cfg80211_calculate_bitrate()

This function contains the HT calculations, which makes no
sense - split that out into a separate function. As a side
effect, this makes the 60G flag independent from HT_MCS so
remove the MCS one from wil6210 (also deleting a duplicate
assignment.)

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