ring: create common function for updating tail index

ring: create common function for updating tail index

Both producer and consumer use the same logic for updating the tail
index so merge into a single function.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>