Racism is the -ism of race — the idea that race is one of the (or the) most important traits of a person.
That through race, you can assume things about them like their worth, how you should treat them, how they act, feel, etc.
The way I see, all of these -isms are all like that (sexism, nationalism, etc).
So no, it’s not racist to call yourself white. It would be racist, though, to say you’re worth more or less than others because of your being white.
Easy solution! Hold your breath ‘till you pass out! :D