ring: Merge common ring_{rx,tx} initialization into own function

ring: Merge common ring_{rx,tx} initialization into own function

Instead of having to perform the individual steps to initialize a ring
and open coding them in multiple places, provide convenience functions
to do all at once. This has the nice side effect of allowing to make
most of these *_{rx,tx}_ring() functions static in their respective
module.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>