[media] dib0700: be sure that dib0700_ctrl_rd() users can do DMA

dib0700_ctrl_rd() takes a RX and a TX pointer. Be sure that
both will point to a memory allocated via kmalloc().

Reviewed-by: Patrick Boettcher <patrick.boettcher@posteo.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>