Input: ads7846 - add possibility to use external vref on ads7846

Just set vref_mv in your platform config to use external vref. Otherwise
the internal one is used.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
diff --git a/include/linux/spi/ads7846.h b/include/linux/spi/ads7846.h
index 92bd083..c64de9d 100644
--- a/include/linux/spi/ads7846.h
+++ b/include/linux/spi/ads7846.h
@@ -14,7 +14,8 @@
 struct ads7846_platform_data {
 	u16	model;			/* 7843, 7845, 7846, 7873. */
 	u16	vref_delay_usecs;	/* 0 for external vref; etc */
-	u16	vref_mv;		/* external vref value, milliVolts */
+	u16	vref_mv;		/* external vref value, milliVolts
+					 * ads7846: if 0, use internal vref */
 	bool	keep_vref_on;		/* set to keep vref on for differential
 					 * measurements as well */
 	bool	swap_xy;		/* swap x and y axes */