s390/qeth: remove gratuitious RX modeset

Trust the IPv4/IPv6 code to properly remove its mcast addresses when a
VLAN device is unregistered, and then also trigger an RX modeset
whenever it's needed.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>