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