media: dvb-usb: Constify struct usb_device_id

'struct usb_device_id' is not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increase overall security.

In order to do that, struct dvb_usb_device_description (in dvb-usb.h) also
needs to be updated.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   1983	   4240	      4	   6227	   1853	drivers/media/usb/dvb-usb/a800.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   2079	   4144	      4	   6227	   1853	drivers/media/usb/dvb-usb/a800.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>