net/smc: rework peer ID handling

This patch initializes the peer ID to a random instance ID and a zero
MAC address. If a RoCE device is in the host, the MAC address part of
the peer ID is overwritten with the respective address. Also, a function
for checking if the peer ID is valid is added. A peer ID is considered
valid if the MAC address part contains a non-zero MAC address.

Signed-off-by: Hans Wippel <ndev@hwipl.net>
Signed-off-by: David S. Miller <davem@davemloft.net>