Minor change for net/mpd5: improve LCP MRU negotiation.

Add a fix from upstream r2246:

Fix bug #62: improve LCP MRU negotiation in some unusual cases
sometimes happening while using Orange S.A. services.
This allow us to setup PPPoE client connection interface using mtu=1492
instead of 1456.

Reported by: Xavier Lemaire
Tested by: Xavier Lemaire