w1: Use linux/gpio.h rather than asm/gpio.h

Direct inclusion of the asm header has long been deprecated by the
introduction of gpiolib.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>