net: ks8851_mll: add ethernet support

The device interface is 16 bits wide.
All the available packets are read from the incoming fifo.

Signed-off-by: Roberto Cerati <roberto.cerati@bticino.it>
Signed-off-by: Raffaele Recalcati <raffaele.recalcati@bticino.it>
[voice.shen@atmel.com: address comments from review results]
[voice.shen@atmel.com: clean up for submit]
Signed-off-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Raffaele Recalcati <raffaele.recalcati@bticino.it>