i2c: virtio: Constify struct i2c_algorithm and struct virtio_device_id

'struct i2c_algorithm' and 'struct virtio_device_id' are 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, which is the case for struct i2c_algorithm.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   6663	    568	     16	   7247	   1c4f	drivers/i2c/busses/i2c-virtio.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   6735	    472	     16	   7223	   1c37	drivers/i2c/busses/i2c-virtio.o

--
Compile tested only

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>