ixgbe: dcb: check setup_tc return codes

dcb netlink code calls setup_tc to init hardware traffic classes
to use for DCB. At some call sites the return values are not
checked for errors and in one case may return -EINVAL back to
the net/dcbnl.c caller which is expecting a u8.

This fixes some smatch hits and although failures are never
seen in practive its best to check return codes.

Reported-by: Dan Carenter <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>