ring: reduce scope of local variables

The local variable i is only used for loop control so define it in
the enqueue and dequeue blocks directly, rather than at the function
level.

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>