Add MTU fallback size of 2048 if detection fails.
Add MTU fallback size of 2048 if detection fails.

If the device doesn't exist before open/1 is called, we won't be able to read the MTU so we now use a fallback.
pvalsecc/enet,archaelus/enet,archaelus/enet,Feuerlabs/enet,pvalsecc/enet