qeth: BRIDGEPORT "sanity check"

Forbid enabling IFF_PROMISC reflection to BRIDGEPORT when a role
is already assigned, and forbid direct manipulation of the role
when reflection mode is engaged.

Reviewed-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Eugene Crosser <Eugene.Crosser@ru.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>