net: dsa: lan9303: correctly check return value of devm_gpiod_get_optional

Function devm_gpiod_get_optional() returns an ERR_PTR on failure. Its
return value should not be validated by a NULL check. Instead, use IS_ERR.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>