ath9k: update the ath_max_4ms_framelen table

Include MCS0-31 and also add SGI for HT20. This makes it
possible to support more different rate combinations with
newer hardware.

Based on a patch by Selvam. T.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>