mpi3mr: Fix MAXPHYS usage

This usage is obsolete. Replace with maximum bus space size. maxphys
will sort itself out at higher levels.

Reviewed by: mav, jhb, imp
Differential Revision: https://reviews.freebsd.org/D42605

(cherry picked from commit 4e6d128bd823e5b5d0b058f918c7036978a0e8bf)