drivers/rtc/rtc-x1205.c: use sign_extend32() for sign extension

Despite its name, sign_extend32() is safe to use for 8 bit types too.
(See https://lkml.org/lkml/2015/1/18/289).
Signed-off-by: NMartin Kepplinger <martink@posteo.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/rtc-x1205.c

@@ -22,6 +22,7 @@
 #include <linux/rtc.h>
 #include <linux/delay.h>
 #include <linux/module.h>
+#include <linux/bitops.h>
 
 #define DRV_VERSION "1.0.8"
 
@@ -366,8 +367,7 @@ static int x1205_get_atrim(struct i2c_client *client, int *trim)
 	 * perform sign extension. The formula is
 	 * Catr = (atr * 0.25pF) + 11.00pF.
 	 */
-	if (atr & 0x20)
-		atr |= 0xC0;
+	atr = sign_extend32(atr, 5);
 
 	dev_dbg(&client->dev, "%s: raw atr=%x (%d)\n", __func__, atr, atr);