staging: gdm724x: fix use after free in gdm_lte_rx()

commit fc7f750d upstream.

The netif_rx_ni() function frees the skb so we can't dereference it to
save the skb->len.

Fixes: 61e12104

 ("staging: gdm7240: adding LTE USB driver")
Cc: stable <stable@vger.kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20220228074331.GA13685@kili
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>