ixgbe: push cls_u32 and mqprio setup_tc processing into separate functions

Let __ixgbe_setup_tc be a splitter for specific setup_tc types and push out
cls_u32 and mqprio specific codes into separate functions. Also change
the return values so they are the same as in the rest of the drivers.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>