staging:iio:magnetometer:ak8975: Don't assume 0 is an invalid GPIO

gpio_is_valid() is the defined mechanism to determine whether a GPIO is
valid. Use this instead of assuming that 0 is an invalid GPIO.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Andrew Chew <achew@nvidia.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>