Fix mac checker regex

Currently, the mac checker regex could match a corner case of
11:22:33:44:55:66: (one extra colon at the end). We fix this, and we
also move the regex compilation outside of this function, at module
level.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>