rtc: m41t80: return NULL rather than a plain 0 integer

Function m41t80_sqw_register_clk returns a pointer to struct clk,
so returning a plain 0 integer isn't good practice. Fix this by
returning a NULL instead.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210925223441.182673-1-colin.king@canonical.com

drivers/rtc/rtc-m41t80.c

@@ -557,7 +557,7 @@ static struct clk *m41t80_sqw_register_clk(struct m41t80_data *m41t80)
 		 * registered automatically when being referenced.
 		 */
 		of_node_put(fixed_clock);
-		return 0;
+		return NULL;
 	}
 
 	/* First disable the clock */