net: thunderx: Fix for crash while BGX teardown

Cortina phy does not have kernel driver and we don't attach
device with phy layer for intefaces like XFI, XLAUI etc,
Hence check for interface type before calling disconnect.

Signed-off-by: Thanneeru Srinivasulu <tsrinivasulu@caviumnetworks.com>
Signed-off-by: Aleksey Makarov <aleksey.makarov@caviumnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>