Check for string length on EUI causes error

Trying to post something to django-rest-framework I get the error `object of type 'EUI' has no len()`. If the mac address gets validated by the EUI constructor anyway, it shouldn't need this validator.

Versions:

Debug toolbar 	1.0.1
Django 	1.6.2
Python 	2.7.5
Rest framework 	2.3.12
South 	0.8.4