MAINTAINERS: remove amd5536udc USB gadget driver maintainer

Thomas can no longer work on the driver, so he asked me to mark the
MAINTAINER entry as "Orphan" with the hope that someone else would
someday pick it up.

Cc: Thomas Dahlmann <dahlmann.thomas@arcor.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>