Input: ads7846 - strict_strtoul takes unsigned long

Fix sparse warning introduced by:
commit 160f1fef7e52e974489b3c70fbd4e094c06965c2 Input: convert drivers to use strict_strtoul()

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
index 6017ea6..d0004dc 100644
--- a/drivers/input/touchscreen/ads7846.c
+++ b/drivers/input/touchscreen/ads7846.c
@@ -472,7 +472,7 @@
 				     const char *buf, size_t count)
 {
 	struct ads7846 *ts = dev_get_drvdata(dev);
-	long i;
+	unsigned long i;
 
 	if (strict_strtoul(buf, 10, &i))
 		return -EINVAL;