[media] rtl28xxu: move usb buffers to state

Buffer needed for USB control message is small so move it to state
and get rid of alloc/free used for each control message.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>