rtc: pcf8563: return -EINVAL when the voltage-low flag is set

Return a proper error code instead of a bare -1 (-EPERM) when VL
indicates the time is not reliable, matching rv3032 and the Linux
driver behaviour.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

drivers/rtc/pcf8563.c

@@ -16,6 +16,7 @@
 #include <log.h>
 #include <rtc.h>
 #include <i2c.h>
+#include <linux/errno.h>
 
 #if !CONFIG_IS_ENABLED(DM_RTC)
 static uchar rtc_read  (uchar reg);
@@ -144,7 +145,7 @@ static int pcf8563_rtc_get(struct udevice *dev, struct rtc_time *tmp)
 
 	if (sec & 0x80) {
 		puts("### Warning: RTC Low Voltage - date/time not reliable\n");
-		rel = -1;
+		rel = -EINVAL;
 	}
 
 	tmp->tm_sec = bcd2bin(sec & 0x7F);