iio:adc:at91-sama5d2: fix vref_uv type

vref_uv has to be an int.
Signed-off-by: NLudovic Desroches <ludovic.desroches@atmel.com>
Reported-by: NJulia Lawall <julia.lawall@lip6.fr>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/iio/adc/at91-sama5d2_adc.c

@@ -171,7 +171,7 @@ struct at91_adc_state {
 	struct clk			*per_clk;
 	struct regulator		*reg;
 	struct regulator		*vref;
-	u32				vref_uv;
+	int				vref_uv;
 	const struct iio_chan_spec	*chan;
 	bool				conversion_done;
 	u32				conversion_value;