emul: Fix endianness fault in sbs gauge emulator

The sbs gauge emulator intercepts SMBus messages for an sbs gauge
compatible IC. However, it was incorrectly interpreting the byte order of
received register write values based on the endianness of the SoC
architecture instead of SMBus' defined little-endianness. This fault was
not caught earlier because native posix and the majority of qemu
architectures use little-endian.

Explicitly interpret the write values as little-endian.

Signed-off-by: Aaron Massey <aaronmassey@google.com>