net: mctp: usb: initialise mac header in RX path

We're not currently setting skb->mac_header on ingress, and the netdev
core rx path expects it. Without it, we'll hit a warning on DEBUG_NETDEV
from commit 1e4033b5 ("net: skb_reset_mac_len() must check if
mac_header was set")

Initialise the mac_header to refer to the USB transport header.

Fixes: 0791c032 ("net: mctp: Add MCTP USB transport driver")
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20250829-mctp-usb-mac-header-v1-1-338ad725e183@codeconstruct.com.au


Signed-off-by: Paolo Abeni <pabeni@redhat.com>