mpi3mr: fix GCC kernel build

Previously every file that included mpi3mr_app.h but did not use
mpi3mr_mgmt_info reported error: 'mpi3mr_mgmt_info' defined but not
used.

Fixes: 2d1d418e1e7b ("mpi3mr: 3rd Generation Tri-Mode NVMe/SAS/SATA...")
Reported by: amd64-gcc12 Cirrus-CI job
Sponsored by: The FreeBSD Foundation