net: fec: enable hardware checksum only on imx6q-fec

Commit 4c09eed (net: fec: Enable imx6 enet checksum acceleration.)
enables hardware checksum acceleration unconditionally for all fec
variants.  This is inappropriate, because some variants like imx5 have
no such support on hardware.  Consequently, fec is broken on these
platforms.  Fix it by enabling hardware checksum only on imx6q-fec type
of controllers.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Jim Baxter <jim_baxter@mentor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>