nvme: Replace potentially long DELAY() with pause().

In some cases like broken hardware nvme(4) may wait minutes for
controller response before timeout. Doing so in a tight spin loop
made whole system unresponsive.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29309
Sponsored by: iXsystems, Inc.

(cherry picked from commit 4fbbe523653b6d2a0186aca38224efcab941deaa)