nvme: Reduce traffic to the doorbell register

Reduce traffic to doorbell register when processing multiple completion
events at once. Only write it at the end of the loop after we've
processed everything (assuming we found at least one completion,
even if that completion wasn't valid).

Sponsored by: Netflix
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D32470

(cherry picked from commit 2ec165e3f065217ae8d54a2a8235fe1f219805ea)