rsi: Add description for function param 'sta'

rsi: Add description for function param 'sta'

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

 drivers/net/wireless/rsi/rsi_91x_mac80211.c:1021: warning: Function parameter or member 'sta' not described in 'rsi_mac80211_set_key'

Cc: Amitkumar Karwar <amitkarwar@gmail.com>
Cc: Siva Rebbagondla <siva8118@gmail.com>
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/20200821071644.109970-3-lee.jones@linaro.org