mpi3mr: Increase device removal handshake queue depth to 16

[chandrakanth.patil_broadcom.com]

Summary

Increase MPI3MR_NUM_DEVRMCMD from 1 to 16 to allow up to 16 concurrent
device removal handshake commands. When multiple target devices are
removed simultaneously (e.g. during cable disconnects or enclosure
unplugs), a single command slot created contention and delayed
handshake processing.

Test Plan

• Clean build with WERROR=-Werror across FreeBSD 16, 15, and 14 with INVARIANTS/WITNESS enabled, git bisect verified.
• Tested simultaneous removal of 16 drives via enclosure power-off during active I/O, verifying concurrent handshake processing and clean teardown.