mwifiex: improve readability in 11ac mcsmap to maxrate conversion

1) rename max_mcs to mcs;
2) initialize 'i' and 'nss' as 1 instead of 0 in nss lookup;
3) use GET_VHTNSSMCS(mcs_map, nss) macro;
4) use IEEE80211_VHT_MCS_* definitions instead of hard coding

Reported-by: Paul Stewart <pstew@chromium.org>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>