mctp i2c: Fix potential use-after-free

The skb is handed off to netif_rx() which may free it.
Found by Smatch.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>