First cut at jumbo frame support. To support large MTU, one or several
separate channels must be allocated to calculate the TCP/UDP checksum
separately, since the mac lacks enough buffers to hold a whole packet
while it's being calculated.

Furthermore, it seems that a single function channel is not quite
enough to feed one of the 10Gig links, so allocate two channels for
XAUI interfaces.
Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Jeff Garzik <jgarzik@pobox.com>