staging: gdm724x: check for buffer overflow in gdm_lte_multi_sdu_pkt()

There needs to be a check to verify that we don't read beyond the end
of "buf".  This function is called from do_rx().  The "buf" is the USB
transfer_buffer and "len" is "urb->actual_length".

Fixes: 61e12104 ("staging: gdm7240: adding LTE USB driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YMcnl4zCwGWGDVMG@mwanda


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>