ring: remove signed type flip-flopping

ring: remove signed type flip-flopping

GCC 8.1 warns:

rte_ring.h:350:46:
warning: conversion to 'uint32_t' {aka 'unsigned int'}
from 'int' may change the sign of the result
[-Wsign-conversion]
  update_tail(&r->prod, prod_head, prod_next, is_sp, 1);

The visible apis take unsigned int, then call a private
api taking an int, which finally calls an api taking an
unsigned int.

Convert the private api to take unsigned int removing
5 x warning similar to that shown above.

Fixes: 0dfc98c507b1 ("ring: separate out head index manipulation")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>