dcbnl: more informed return values for new dcbnl routines

More accurate return values for the following (new) dcbnl routines:
dcbnl_getdcbx()
dcbnl_setdcbx()
dcbnl_getfeatcfg()
dcbnl_setfeatcfg()

Signed-off-by: Shmulik Ravid <shmulikr@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>