board_r: return 0 from show_model_r

The show_model_r function should return an int but didn't. Return 0 to
indicate inevitable success and avoid the following if it is used:

common/board_r.c: In function 'show_model_r':
common/board_r.c:531:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Simon Glass <sjg@chromium.org>