media: em28xx: constify most static structs

There are several em28xx static structs that can now be constified.

That caused a significant reduction at data segment:

Before:
   text	   data	    bss	    dec	    hex	filename
  85017	  59588	    576	 145181	  2371d	drivers/media/usb/em28xx/em28xx.o

After:
   text	   data	    bss	    dec	    hex	filename
 112345	  32292	    576	 145213	  2373d	drivers/media/usb/em28xx/em28xx.o

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