media: dibx000_common: Constify struct i2c_algorithm

'struct i2c_algorithm' is not modified in this driver.

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

To do so, the prototype of i2c_adapter_init() has to be updated as well.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  17213	    932	     20	  18165	   46f5	drivers/media/dvb-frontends/dibx000_common.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  17490	    660	     20	  18170	   46fa	drivers/media/dvb-frontends/dibx000_common.o

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