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

drivers/net/wireless/rsi/rsi_91x_mac80211.c

@@ -1009,6 +1009,7 @@ static int rsi_hal_key_config(struct ieee80211_hw *hw,
  * @hw: Pointer to the ieee80211_hw structure.
  * @cmd: enum set_key_cmd.
  * @vif: Pointer to the ieee80211_vif structure.
+ * @sta: Pointer to the ieee80211_sta structure.
  * @key: Pointer to the ieee80211_key_conf structure.
  *
  * Return: status: 0 on success, negative error code on failure.