MIPS: Octeon: Fix to IP checksum offloading in Little Endian

When hardware checksum generation is switched on the checksum
generation was only being signalled to the hardware correctly
in Big Endian mode.

Signed-off-by: Paul Martin <paul.martin@codethink.co.uk>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9634/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>