net: phy: sfp: warn the user when no tx_disable pin is available

In case no Tx disable pin is available the SFP modules will always be
emitting. This could be an issue when using modules using laser as their
light source as we would have no way to disable it when the fiber is
removed. This patch adds a warning when registering an SFP cage which do
not have its tx_disable pin wired or available.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>