hwmon: (max6621) Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115


Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>