[media] b2c2: constify nxt200x_config structure

Declare nxt200x_config structure as const as it is 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 case is nxt200x_attach and the parameter of
this function to which the object reference is 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
   7566	    568	      0	   8134	   1fc6	common/b2c2/flexcop-fe-tuner.o

File size after:
   text	   data	    bss	    dec	    hex	filename
   7582	    536	      0	   8118	   1fb6	common/b2c2/flexcop-fe-tuner.o

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