w1-gpio: Detect of_gpio_error for first gpio

The first DT gpio is necessary for this driver, but errors returned for
of_get_gpio are ignored.

This patch adds a return value check for the first of_get_gpio.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>