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 <email@example.com>
Reviewed by: luporl, Sreekanth Reddy <firstname.lastname@example.org> (by e-mail)
Sponsored by: Eldorado Research Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26040