crypto: qat - fix seq_file position update in adf_ring_next()

The `adf_ring_next()` function in the QAT debug transport interface
fails to correctly update the position index when reaching the end of
the ring elements. This triggers the following kernel warning when
reading ring files, such as
/sys/kernel/debug/qat_c6xx_<D:B:D:F>/transport/bank_00/ring_00:

   [27725.022965] seq_file: buggy .next function adf_ring_next [intel_qat] did not update position index

Ensure that the `*pos` index is incremented before returning NULL when
after the last element in the ring is found, satisfying the seq_file API
requirements and preventing the warning.

Fixes: a672a9dc ("crypto: qat - Intel(R) QAT transport code")
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>