[media] saa7134: constify nxt200x_config structures

Declare nxt200x_config structures as const as they are only passed as
an argument to the function dvb_attach. dvb_attach calls its first
argument on the rest of its arguments. The first argument of
dvb_attach in the changed cases is nxt200x_attach and the parameter of
this function to which the object references are passed is of type
const. So, nxt200x_config structures having this property can be made
const.

File size before:
   text	   data	    bss	    dec	    hex	filename
  21320	   3776	     16	  25112	   6218	saa7134/saa7134-dvb.o

File size after:
   text	   data	    bss	    dec	    hex	filename
  21384	   3744	     16	  25144	   6238	saa7134/saa7134-dvb.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>