This is currently missing, which results in a crash when one attempts
to set VXLAN tunnel over the mlx4_en when acting as PF.

	[ 2408.785472] BUG: unable to handle kernel NULL pointer dereference at (null)
	[...]
	[ 2408.994104] Call Trace:
	[ 2408.996584]  [<ffffffffa021f7f5>] ? vxlan_get_rx_port+0xd6/0x103 [vxlan]
	[ 2409.003316]  [<ffffffffa021f71f>] ? vxlan_lowerdev_event+0xf2/0xf2 [vxlan]
	[ 2409.010225]  [<ffffffffa0630358>] mlx4_en_start_port+0x862/0x96a [mlx4_en]
	[ 2409.017132]  [<ffffffffa063070f>] mlx4_en_open+0x17f/0x1b8 [mlx4_en]

While here, make sure to invoke vxlan_get_rx_port() only when VXLAN
offloads are actually enabled and not when they are only supported.
Reported-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>