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 <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/rtc/rtc-m41t80.c

···
 		 * registered automatically when being referenced.
 		 */
 		of_node_put(fixed_clock);
-		return 0;
+		return NULL;
 	}
 
 	/* First disable the clock */