i2c: gpio: OF gpio code does not handle defered probe case

When using device-tree and the i2c-gpio driver is called before the
GPIO node has been probed then it needs to correctly defer the probe
instead of returning a permanent error that the gpio numbers are not
valid.

This fixes the following error:
	/i2c@2: invalid GPIO pins, sda=-517/scl=-517

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-gpio.c

@@ -94,6 +94,9 @@ static int of_i2c_gpio_get_pins(struct device_node *np,
 	*sda_pin = of_get_gpio(np, 0);
 	*scl_pin = of_get_gpio(np, 1);
 
+	if (*sda_pin == -EPROBE_DEFER || *scl_pin == -EPROBE_DEFER)
+		return -EPROBE_DEFER;
+
 	if (!gpio_is_valid(*sda_pin) || !gpio_is_valid(*scl_pin)) {
 		pr_err("%s: invalid GPIO pins, sda=%d/scl=%d\n",
 		       np->full_name, *sda_pin, *scl_pin);