ath6kl: wmi: Remove unused variable 'rate'

Fixes the following W=1 kernel build warning(s):

 drivers/net/wireless/ath/ath6kl/wmi.c: In function ‘ath6kl_wmi_bitrate_reply_rx’:
 drivers/net/wireless/ath/ath6kl/wmi.c:1204:6: warning: variable ‘rate’ set but not used [-Wunused-but-set-variable]

Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200826093401.1458456-15-lee.jones@linaro.org