altera_tse: Correct rx packet length

Altera TSE MAC rx DMA transfer starts with the 2 additional bytes for IP
payload alignment. This patch fixes tse_rx() function loop which reads DMA
rx status and extracts packet length from it. Status signalises a whole DMA
transfer length, which is 2 bytes longer than the packet itself.

Signed-off-by: Vlastimil Setka <setka@vsis.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>