ibmvnic: fix accelerated VLAN handling

[ Upstream commit e84b47941e15e6666afb8ee8b21d1c3fc1a013af ]

Don't request tag insertion when it isn't present in outgoing skb.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>