ring: fix return type in enqueue and dequeue burst functions

Enqueue and dequeue burst functions always return a positive
value (including 0), so return type should be unsigned,
instead of int.

Fixed also API doc for one of the functions.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>