Fix queue length reporting in mps(4) and mpr(4).

Both drivers were found to report CAM bigger queue depth then they really
can handle. It made them later under high load with many disks return
some of submitted requests back with CAM_REQUEUE_REQ status for later
resubmission.

Reviewed by: scottl
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D14215