rtc: abx80x: Fix return value of nvmem callback on read

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

abx80x_nvmem_xfer() on read calls i2c_smbus_read_i2c_block_data() which
returns the number of bytes read on success as per its api description,
this return value is handled as an error and returned to nvmem even on
success.

Fix to handle all possible values that would be returned by
i2c_smbus_read_i2c_block_data().

Fixes: e90ff8ede777 ("rtc: abx80x: Add nvmem support")
Cc: [e-mail]
Signed-off-by: Joy Chakraborty <[e-mail]>
Reviewed-by: Dan Carpenter <[e-mail]>
Reviewed-by: Sean Anderson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-abx80x.c

···
 		if (ret)
 			return ret;
 
-		if (write)
+		if (write) {
 			ret = i2c_smbus_write_i2c_block_data(priv->client, reg,
 							     len, val);
-		else
+			if (ret)
+				return ret;
+		} else {
 			ret = i2c_smbus_read_i2c_block_data(priv->client, reg,
 							    len, val);
-		if (ret)
-			return ret;
+			if (ret <= 0)
+				return ret ? ret : -EIO;
+			len = ret;
+		}
 
 		offset += len;
 		val += len;