ipg: always compile in jumbo frame support

Add a ->is_jumbo boolean to struct ipg_nic_private and fix up
ipg_interrupt_handler() to call the jumbo frame version of ipg_nic_rx() if the
boolean is set to true. Also remove the JUMBO_FRAME #ifdefs so we can always
compile in support for jumbo frames.

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