[media] rtl28xxu: change module unregister order

We must unregister frontend first and after that driver itself. That
order went wrong after demod drivers were switched to kernel I2C
drivers, causing crashes.

Tested-by: Benjamin Larsson <benjamin@southpole.se>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>