media: i2c: tvp5150: Constify some structures

'vbi_ram_default' and 'tvp5150_config' are not modified in this diver
and are only used as a const struct.

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

On a x86_64, with allmodconfig:
Before:
   text	   data	    bss	    dec	    hex	filename
  57197	   2936	     36	  60169	   eb09	drivers/media/i2c/tvp5150.o

After:
   text	   data	    bss	    dec	    hex	filename
  57517	   2608	     36	  60161	   eb01	drivers/media/i2c/tvp5150.o

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