net: mvneta: Fix big endian issue in mvneta_txq_desc_csum()

This commit fixes the command value generated for CSUM calculation
when running in big endian mode.  The Ethernet protocol ID for IP was
being unconditionally byte-swapped in the layer 3 protocol check (with
swab16), which caused the mvneta driver to not function correctly in
big endian mode.  This patch byte-swaps the ID conditionally with
htons.

Cc: <stable@vger.kernel.org> # v3.13+
Signed-off-by: Thomas Fitzsimmons <fitzsim@fitzsim.org>
Signed-off-by: David S. Miller <davem@davemloft.net>