lan743x: trim all 4 bytes of the FCS; not just 2

Trim all 4 bytes of the received FCS; not just 2 of them. Leaving 2
bytes of the FCS on the frame breaks DSA tailing tag drivers.

Fixes: a8db76d4

 ("lan743x: boost performance on cpu archs w/o dma cache snooping")
Signed-off-by: George McCollister <george.mccollister@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>