staging:iio:magnetometer:ak8975 use platform_data to pass the gpio number.

Tegra doesn't have irq_to_gpio() any more, and ak8975 is included in
tegra_defconfig. This causes a build failure. Instead, pass the GPIO name
through platform data.

[swarren: Rewrote commit description when I squashed this with my patch
to remove the irq_to_gpio() call]

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