mpi3mr: Use template for main busdma tag.

Use the simpler template code for the parent busdma tag for all I/O to
this card.

Reviewed by: mav, jhb, imp
Differential Revision: https://reviews.freebsd.org/D42607

(cherry picked from commit 489eee0d41dce317678adb0dae8d509a5f1d6d93)