MAINTAINERS: usb251xb: remove reference inexistent file

The platform_data header file was dropped in the merged version of the
USB251xB driver. Therefore remove its reference from the MAINTAINERS file.

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>