net: usb: qmi_wwan: fix memory leak for not ip packets

[ Upstream commit 7ab107544b777c3bd7feb9fe447367d8edd5b202 ]

Free the unused skb when not ip packets arrive.

Fixes: c6adf779 ("net: usb: qmi_wwan: add qmap mux protocol support")
Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>