sfc: optional logging of TC offload errors

TC offload support will involve complex limitations on what matches and
 actions a rule can do, in some cases potentially depending on rules
 already offloaded.  So add an ethtool private flag "log-tc-errors" which
 controls reporting the reasons for un-offloadable TC rules at NETIF_INFO.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>