net: phy: phylink: Don't release NULL GPIO

If CONFIG_GPIOLIB is disabled, gpiod_put() becomes a stub that produces a
warning, this helped identify that we could be attempting to release a NULL
pl->link_gpio GPIO descriptor, so guard against that.

Fixes: daab3349 ("net: phy: phylink: Release link GPIO")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>