ftgmac100: Simplify rx pointer handling in the rx path

We don't handle fragmented RX packets, so the "looping"
helpers to locate the first segment of a packet or to
drop a packet aren't actually helping.

Take them out and simplify ftgmac100_rx_packet() further
as a result.

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