ath6kl: Add support for setting tx rateset.

Tx legacy and mcs rateset can configured using iw for
2.4 and 5 bands.  Add support for the same in driver.

kvalo: add an enum for the hw flags and rename the flag accordingly,
rename ath6kl_cfg80211_set_bitrate_mask() to a shorter version to make
it easier to indent

Signed-off-by: Bala Shanmugam <bkamatch@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>