rtw88: assign NULL to skb after being kfree()'ed

Should assign NULL to skb after kfree(), in case of driver
trying to free the same skb again.

This could happen if driver failed to allocate an skb when
building reserved page.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>