octeontx2-af: Enable LBK links only when switch mode is on.

Currently, all the TL3_TL2 nodes are being configured to enable
switch LBK channel 63 in them. Instead enable them only when switch
mode is enabled.

Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Naveen Mamindlapalli <naveenm@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>