w1-gpio: Simplify & get rid of defines

There's no reason to have the OF defines; it complicates the driver.
There's also no need for the funky platform_driver_probe.

Add a few warnings in case there's a failure; helps us find out
what went wrong.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>