mips: Use little endian packet accessors.

mips: Use little endian packet accessors.

We know data is little endian, so we can use those accessors
explicitly.

Change-Id: I6220a543686b1d45d26973391b028dc04ce85dd2
Reviewed-on: https://gem5-review.googlesource.com/c/13460
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>