ath9k: Fix incorrect user ratekbs of MCS15 ShortGI

The user ratekbs of MCS15 ShortGI is incorrect and can not be lesser
than MCS15 rate. This incorrect rate may affect switching to higher
rates as the rate control algorithm always finds MCS15 is better
than MCS15 ShortGI and results in lower throughput. Fix this by
feeding the correct user ratekbs for MCS15 ShortGI rate.

This issue affects 3 stream case very badly as the 3 stream rates are
not used at all once we scale down to MCS15 from 3 stream rates.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>