iio: adc: rcar-gyroadc: Cast pointer to uintptr_t to fix warning on 64-bit

On 64-bit:

    drivers/iio/adc/rcar-gyroadc.c: In function 'rcar_gyroadc_parse_subdevs':
    drivers/iio/adc/rcar-gyroadc.c:352:15: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
       childmode = (unsigned int)of_id->data;
		   ^

Cast the pointer to uintptr_t instead of unsigned int to fix this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>