regulator: gpio: check return value of of_get_named_gpio

At boot time the regulator driver can be initialized before the
gpio, in which case the call to of_get_named_gpio will return
EPROBE_DEFER. This value is silently passed to regulator_register
which will return success, although the gpio is not registered
(regulator_ena_gpio_request not called) as the value passed is
detected as invalid. The gpio_regulator_probe will therefore
succeed win no gpio requested.

Signed-off-by: Mihai Mihalache <mihai.d.mihalache@intel.com>
Reviewed-by: Hans Holmberg <hans.holmberg@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>