media: dvb-usb: Constify struct i2c_algorithm

'struct i2c_algorithm' are not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers.

More over, dvb_usb_device_properties->i2c_algo seems to only be copied in
i2c_adapter->algo, which is already a "const struct i2c_algorithm".
This is done in dvb_usb_i2c_init()

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  30571	   5916	     36	  36523	   8eab	drivers/media/usb/dvb-usb/af9005.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  30667	   5852	     36	  36555	   8ecb	drivers/media/usb/dvb-usb/af9005.o

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