ixgbe: don't clear UDP tunnel ports when RXCSUM is disabled

It appears the clearing of UDP tunnel ports when RXCSUM
is disabled is unnecessary. Driver will not pay attention
to checksum bits if RXCSUM is not set, so we can let
the hardware parse the packets.

Note that the UDP tunnel port NDO handlers don't pay attention
to the state of RXCSUM, so the ports could had been re-programmed,
anyway.

This cleanup simplifies later conversion patch.

v2:
 - break this out of the following patch.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>