ipg: per-device rxfrag_size

Add a ->max_rxframe member to struct ipg_nic_private and convert the users of
IPG_RXFRAG_SIZE to use it instead to enable per-device jumbo frame
configuration.

Tested-by: Andrew Savchenko <Bircoph@list.ru>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>