net: mvpp2: Make TX / RX descriptors little-endian

The PPv2 controller always expect descriptors to be in little endian. We
must therefore force descriptors to use that format, and convert to the
host endianness when necessary.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>