rt2x00: update MAINTAINERS

Although I have always been the active maintainer of the rt2x00 drivers,
I was not mentioned explicitely in the MAINTAINERS file as such.

Update the rt2x00 entry in the MAINTAINERS file to add my name and
email address.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>