net/macb: change RX path for GEM

GEM is able to adapt its DMA buffer size, so change
the RX path to take advantage of this possibility and
remove all kind of memcpy in this path.
This modification introduces function pointers for managing
differences between MACB and GEM adapter type.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>