mwl8k: remove nonstandard rate 72 Mbps

This rate causes an overflow in the extended rates IE's data rate field,
with the overflowing bit setting the Basic Rate Set membership. This
results in a bogus 8 Mpbs basic rate, making clients checking them refuse
association.

Since the rate is likely unused anyway (HT will yield better rates between
supporting chips), we can just remove it.

This fixes association from wpa_supplicant and Android 4.x and newer.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>