ath9k_htc: Add a new WMI command to set a rate mask

This patch adds WMI_BITRATE_MASK_CMDID which can be
used by the set_bitrate_mask() handler.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>