A bit of a tough one that would have to be decided on legality. It was in her yard not on her patrol car. But does the police department have the right to require officers to uphold a standard that presents neutrality to the public in all things. It is freedom of speech but I'd guess an employer can require employees to not wear say a confederate flag shirt to work or any confederate flag patch or emblem as it would be offensive to other employees and customers.
And what if it were a KKK flag, or an Isis flag, or an old Soviet Union flag. One for the lawyers and the judges.