w1-gpio: fix section mismatch

This fixes the following section mismatch:

WARNING: drivers/w1/masters/w1-gpio.o(.data+0x188): Section mismatch in
reference from the variable w1_gpio_driver to the function
.init.text:w1_gpio_probe()
The variable w1_gpio_driver references
the function __init w1_gpio_probe()
If the reference is valid then annotate the
variable with __init* or __refdata (see linux/init.h) or name the variable:
*_template, *_timer, *_sht, *_ops, *_probe, *_probe_one, *_console

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>