Skip setting the MTU for ENA if it is not changing

On AWS, a network interface can get reinitialized every 30 minutes due
to the MTU being (re)set when a new DHCP lease is obtained. This can
cause packet drop, along with annoying syslog messages.

Skip setting the MTU in the ena driver if the new MTU is the same as the
old MTU. Note this fix is already in the netfront driver.

Testing: Verified ena up/down messages do not appear every 30 min in
/var/log/messages with the fix in place.

Submitted by: Krishna Yenduri <kyenduri@brkt.com>
Reviewed by: Michal Krawczyk <mk@semihalf.com>