neta: Fix MTU change sequence

The IFF_DRV_RUNNING flag is used to see if the interface needs
to be temporarily brought down during MTU change sequence.
The problem here is that this flag is cleared in mvneta_stop_locked,
resulting in the reinitialization logic never being executed after
MTU has been changed.
Fix that by saving the flag value before the interface is brought down.

Reported by: Jérôme Tomczyk <jerome.tomczyk@stormshield.eu>
Approved by: mw(mentor)
Obtained from: Semihalf
Sponsored by: Stormshield
MFC after: 2 weeks