IB/ipoib: Consolidate checking of the proposed child interface

Move all the checking for pkey and other validity to the __ipoib_vlan_add
function. This removes the last difference from the control flow
of the __ipoib_vlan_add to make the overall design simpler to
understand.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>