net: dsa: lan9303: check error value from devm_gpiod_get_optional()

devm_gpiod_get_optional() can return an error in addition to a NULL ptr.
Check for error and propagate that to the probe function. Check return
value in probe. This will now handle EPROBE_DEFER for the reset gpio.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>