[media] si4713: Added MAINTAINERS entry for radio-usb-si4713 driver

Hans Verkuil <hverkuil@xs4all.nl> will maintain the USB driver for si4713

Signed-off-by: Dinesh Ram <dinesh.ram@cern.ch>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Eduardo Valentin <edubezval@gmail.com>
Acked-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>