[media] flexcop-usb: don't use stack for DMA

The USB control messages require DMA to work. We cannot pass
a stack-allocated buffer, as it is not warranted that the
stack would be into a DMA enabled area.

While here, remove a dead function calling usb_control_msg().

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>