mpr: big-endian support

This fixes mpr driver on big-endian devices.
Tested on powerpc64 and powerpc64le targets using a SAS9300-8i card
(LSISAS3008 pci vendor=0x1000 device=0x0097)

Submitted by: Andre Fernando da Silva <andre.silva@eldorado.org.br>
Reviewed by: luporl, alfredo, Sreekanth Reddy <sreekanth.reddy@broadcom.com> (by email)
Sponsored by: Eldorado Research Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25785

(cherry picked from commit 71900a794da046ad5322caae2774aed5b3d361b9)