ftgmac100: Drop support for fragmented receive

We don't support jumbo frames, we will never receive a
fragmented packet, the RX buffer is always big enough,
if not then it's a runaway packet that can be dropped.

So take out the loop that handles such things in
ftgmac100_rx_packet() which will help with subsequent
simplifications and improvements to the RX path

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>