media: dvb-usb-v2: Constify struct i2c_algorithm

media: dvb-usb-v2: 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_usbv2_i2c_init()

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  35366	   5832	     36	  41234	   a112	drivers/media/usb/dvb-usb-v2/af9015.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  35430	   5768	     36	  41234	   a112	drivers/media/usb/dvb-usb-v2/af9015.o

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