ring: fix sign conversion warning

ring: fix sign conversion warning

On gcc 5.4.0 / native aarch64 from Ubuntu 16.04:

In function '__rte_ring_do_dequeue':
rte_ring.h: 385:35: warning:
conversion to 'int' from 'unsigned int' may change
the sign of the result [-Wsign-conversion]
  n = __rte_ring_move_cons_head(r, is_sc, n, behavior,
                                   ^

Fixes: e8ed5056c8 ("ring: remove signed type flip-flopping")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>