[ENHANCEMENT] Add validation of least significant (mcast) via luci

If an 'invalid' mac is being set via luci, the mac will be set but not actually applied to the interfaces. If we use generators, like: https://www.macvendorlookup.com/mac-address-generator or https://miniwebtool.com/de/mac-address-generator/ they might generate 'invalid' mac addresses and the user is not informed about it, assuming it is a bug.