wireless: report reasonable bitrate for MCS rates through wext

Previously, cfg80211 had reported "0" for MCS (i.e. 802.11n) bitrates
through the wireless extensions interface.  However, nl80211 was
converting MCS rates into a reasonable bitrate number.  This patch moves
the nl80211 code to cfg80211 where it is now shared between both the
nl80211 interface and the wireless extensions interface.

Signed-off-by: John W. Linville <linville@tuxdriver.com>