nvme: limit I/O queue trackers to Identify MAXCMD

MAXCMD is the maximum number of commands the controller processes at
one time for a particular I/O queue. Allocating more trackers than
that only queues commands inside the controller. Controllers
reporting 0 are unaffected.

Approved by: ngie (co-mentor)

Reviewed by: ngie, imp
Differential Revision: https://reviews.freebsd.org/D59154