mvneta: Fix MTU update sequence

After MTU is updated we might start using allocating RX buffers from different pool. (MJUM9BYTES vs MCLBYTES)
Because of that we need to update the RX buffer size in hardware.
Previously it was done only when the interface was up, which is incorrect since MTU can be changed at any time.

Differential revision: https://reviews.freebsd.org/D31724
Sponsored by: Stormshield
Obtained from: Semihalf
MFC after: 2 weeks
Reviewed by: wma