neta: Fix MTU change sequence

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

(cherry picked from commit 97ecdc00ac5ac506f4119be9570d13de2d3a003a)