net: fec: make driver endian-safe

The driver treats the device descriptors as CPU-endian, which appears
to be correct with the default endianness on both ARM (typically LE)
and PowerPC (typically BE) SoCs, indicating that the hardware block
is generated differently. Add endianness annotations and byteswaps as
necessary.

It's not clear that the ifdef there really is correct and shouldn't
just be #ifdef CONFIG_ARM, but I also can't test on anything but the
i.MX6 HummingBoard where this gets it working with a BE kernel.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>