mctp: i2c: fix skb memory leak in receive path

When 'midev->allow_rx' is false, the newly allocated skb isn't consumed
by netif_rx(), it needs to free the skb directly.

Fixes: f5b8abf9fc3d ("mctp i2c: MCTP I2C binding driver")
Signed-off-by: Haiyue Wang <haiyuewa@163.com>
Link: https://patch.msgid.link/20260305143240.97592-1-haiyuewa@163.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>