The code to change MTU doesn't correctly handle all the chip variations
and requirements for restarting.  On Genesis chips changing MTU would just
cause receiver to hang.
Use a simpler approach of just taking link down/up if needed.
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>