fff-network: Remove MTU 1528 from ethmesh device

This interface was using 1500 anyway and setting ethernet to
MTU > 1500 might not be a good idea.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Robert Langammer <rlanghammer@web.de>