rtl8xxxu: prevent potential memory leak

Free the skb if usb_submit_urb fails on rx_urb. And free the urb
no matter usb_submit_urb succeeds or not in rtl8xxxu_submit_int_urb.

Signed-off-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200906040424.22022-1-chiu@endlessm.com