ath9k: Enable Short GI in 20 Mhz for ar9287 and later chips

This patch enables short GI rx at all rates and tx at mcs15
for 20 Mhz channel width also.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>