ipg: remove jumbo frame #ifdef from mtu

Remove JUMBO_FRAME #ifdef from dev->mtu setting in ipg_nic_open() so that we
can make IPG_TXFRAG_SIZE configurable.

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