mwifiex: make 11ac mcs rate tables global and const

Remove these local array variables and define them as static
const array in global space.
The duplicated mcs_rate table is removed automatically with this
change.

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