rtc: pcf8563: fix write of invalid bits to ST2 reg

The NXP datasheet says:
 "Bits labeled as N should always be written with logic 0."

At least one of those bits is sometime read as a 1, therfore violating
this rule.  To fix this we mask away those bits.
Signed-off-by: Jan Kardell <jan.kardell@telliq.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Vincent Donnefort <vdonnefort@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/rtc-pcf8563.c

@@ -28,6 +28,7 @@
 #define PCF8563_REG_ST2		0x01
 #define PCF8563_BIT_AIE		(1 << 1)
 #define PCF8563_BIT_AF		(1 << 3)
+#define PCF8563_BITS_ST2_N	(7 << 5)
 
 #define PCF8563_REG_SC		0x02 /* datetime */
 #define PCF8563_REG_MN		0x03
@@ -130,7 +131,7 @@ static int pcf8563_set_alarm_mode(struct i2c_client *client, bool on)
 	else
 		buf &= ~PCF8563_BIT_AIE;
 
-	buf &= ~PCF8563_BIT_AF;
+	buf &= ~(PCF8563_BIT_AF | PCF8563_BITS_ST2_N);
 
 	err = pcf8563_write_block_data(client, PCF8563_REG_ST2, 1, &buf);
 	if (err < 0) {