HID: hid-lg: make array cbuf static const to shink object code size

Don't populate array cbuf on the stack, instead make it static.
Makes the object code smaller by over 110 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  15096	   3504	    128	  18728	   4928	drivers/hid/hid-lg.o

After:
   text	   data	    bss	    dec	    hex	filename
  14884	   3600	    128	  18612	   48b4	drivers/hid/hid-lg.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>