net: ll_temac: fix checksum offload logic

The current checksum offload code does not work and this corrects
that functionality. It also updates the interrupt coallescing
initialization so than there are fewer interrupts and performance
is increased.

Signed-off-by: Brian Hill <brian.hill@xilinx.com>
Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>