w1-gpio: handle of_get_gpio() returning -EPROBE_DEFER better

of_get_gpio() might return -EPROBE_DEFER meaning that the driver
providing the gpio isn't ready yet. If that happens for the first gpio
the resulting kernel output without this patch is:

	w1-gpio somename: Failed to parse DT
	platform somename: Driver w1-gpio requests probe deferral

The first message is misleading and so is suppressed with this patch.

Further if determining the gpio to switch the external pullup yields
-EPROBE_DEFER this error should be passed back to the caller instead of
just continuing without pullup.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>