wifi: rtw89: free unused skb to prevent memory leak

This avoid potential memory leak under power saving mode.

Signed-off-by: Po-Hao Huang <phhuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220916033811.13862-6-pkshih@realtek.com