skge: MTU changing fix

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>