Previously the bridge MTU was set to ETHERMTU and could not be changed. Since
we can only bridge interfaces with the same value it meant that all members had
to be set at ETHERMTU as well.

Allow the first member to be added to define the MTU for the bridge, the check
still applies to all additional members.

Print an informative message if the MTU is incorrect [1]

Requested by: Niki Denev [1]
Approved by: mlaier (mentor)
MFC after: 3 days