vrf: ndo_add|del_slave drop unnecessary checks

When ndo_add|del_slave ops are used, they're taken from the respective
master device's netdev ops, so if the master device is a VRF only then
the VRF ops will get called thus no need to check the type of the
master.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>