ftgmac100: Disable HW checksum generation on AST2400, enable on others

We found out that HW checksum generation only works from AST2500
onward. This disables it on AST2400 and removes the "no-hw-checksum"
properties in the device-trees. The problem we had wasn't related
to NC-SI.

Also rework the logic testing for that property so it can be used
to disable HW checksum generation and checking regardless of whether
NC-SI is used or not in case other variants out there need this.

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