ixgbe: fix smatch splat due to missing NULL check

ixgbe_ieee_ets and ixgbe_ieee_pfc are intialized at
the same time. Do a check for both before configuring
IEEE802.1Qaz. Also max_frame was causing a sparse
warning resolved here as well.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>