rtc: m41t80: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Notice that in this particular case, I replaced "Fall" with a proper
"Fall through" comment.

Addresses-Coverity-ID: 1373875 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>