crypto: qat - refactor ring-related debug functions

Refactor the functions `adf_ring_start()` and `adf_ring_next()` to
improve readability.

This does not introduce any functional change.

Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Ahsan Atta <ahsan.atta@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>