Input: ads7846 - add support for AD7843 parts

The AD7873 is almost identical to the ADS7846; the only difference is
related to the Power Management bits PD0 and PD1.  This results in a
slightly different PENIRQ enable behavior.  For the AD7873, VREF should
be turned off during differential measurements.

So, add the AD7873/43 to the list of driver supported devices, and prevent
VREF usage during differential/ratiometric conversion modes.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
diff --git a/include/linux/spi/ads7846.h b/include/linux/spi/ads7846.h
index 51948eb..5710c15 100644
--- a/include/linux/spi/ads7846.h
+++ b/include/linux/spi/ads7846.h
@@ -12,7 +12,7 @@
 };
 
 struct ads7846_platform_data {
-	u16	model;			/* 7843, 7845, 7846. */
+	u16	model;			/* 7843, 7845, 7846, 7873. */
 	u16	vref_delay_usecs;	/* 0 for external vref; etc */
 	u16	vref_mv;		/* external vref value, milliVolts */
 	bool	keep_vref_on;		/* set to keep vref on for differential