rtc: cmos: Fix return value of nvmem callbacks

Read/write callbacks registered with nvmem core expect 0 to be returned
on success and a negative value to be returned on failure.

cmos_nvram_read()/cmos_nvram_write() currently return the number of
bytes read or written, fix to return 0 on success and -EIO incase number
of bytes requested was not read or written.

Fixes: 8b5b7958 ("rtc: cmos: use generic nvmem")
Cc: stable@vger.kernel.org
Signed-off-by: Joy Chakraborty <joychakr@google.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20240612083635.1253039-1-joychakr@google.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-cmos.c

@@ -643,11 +643,10 @@ static int cmos_nvram_read(void *priv, unsigned int off, void *val,
 			   size_t count)
 {
 	unsigned char *buf = val;
-	int	retval;
 
 	off += NVRAM_OFFSET;
 	spin_lock_irq(&rtc_lock);
-	for (retval = 0; count; count--, off++, retval++) {
+	for (; count; count--, off++) {
 		if (off < 128)
 			*buf++ = CMOS_READ(off);
 		else if (can_bank2)
@@ -657,7 +656,7 @@ static int cmos_nvram_read(void *priv, unsigned int off, void *val,
 	}
 	spin_unlock_irq(&rtc_lock);
 
-	return retval;
+	return count ? -EIO : 0;
 }
 
 static int cmos_nvram_write(void *priv, unsigned int off, void *val,
@@ -665,7 +664,6 @@ static int cmos_nvram_write(void *priv, unsigned int off, void *val,
 {
 	struct cmos_rtc	*cmos = priv;
 	unsigned char	*buf = val;
-	int		retval;
 
 	/* NOTE:  on at least PCs and Ataris, the boot firmware uses a
 	 * checksum on part of the NVRAM data.  That's currently ignored
@@ -674,7 +672,7 @@ static int cmos_nvram_write(void *priv, unsigned int off, void *val,
 	 */
 	off += NVRAM_OFFSET;
 	spin_lock_irq(&rtc_lock);
-	for (retval = 0; count; count--, off++, retval++) {
+	for (; count; count--, off++) {
 		/* don't trash RTC registers */
 		if (off == cmos->day_alrm
 				|| off == cmos->mon_alrm
@@ -689,7 +687,7 @@ static int cmos_nvram_write(void *priv, unsigned int off, void *val,
 	}
 	spin_unlock_irq(&rtc_lock);
 
-	return retval;
+	return count ? -EIO : 0;
 }
 
 /*----------------------------------------------------------------*/