net/smc: introduce list of pnetids for Ethernet devices

SMCD version 2 allows usage of ISM devices with hardware PNETID
only, if an Ethernet net_device exists with the same hardware PNETID.
This requires to maintain a list of pnetids belonging to
Ethernet net_devices, which is covered by this patch.

Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>