mprutil: add big endian support

This fix mprutil on big endian platforms, as follow up of D25785.

Flash operations are still not working, such as MPI2_FUNCTION_FW_UPLOAD
failing due to timeout.

Firmware version used during tests: 16.00.01.00

Submitted by: Andre Fernando da Silva <andre.silva@eldorado.org.br>
Reviewed by: luporl, Sreekanth Reddy <sreekanth.reddy@broadcom.com> (by e-mail)
Sponsored by: Eldorado Research Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26040

(cherry picked from commit fc9780fd41e7b29db04e9cb843cb5034b88ae662)