MAINTAINERS: change rt2x00 maintainer

After short chat with Ivo, we decided that I'll take maintenance
of rt2x00 driver.

Thanks for Ivo's great work in the past!

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Ivo Van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>