nl80211/cfg80211: add VHT MCS support

Add support for reporting and calculating VHT MCSes.

Note that I'm not completely sure that the bitrate
calculations are correct, nor that they can't be
simplified.

Change-Id: Id4c132850a85ff59f0fc16396763ed717689bec0
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: db9c64cf8d9d3fcbc34b09d037f266d1fc9f928c
Git-repo:
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>