hwmon: (f75375s) Fix warning message seen in some configurations

In some configurations, BUG() does not result in an endless loop but returns to
the caller. This results in the following compiler warning:

drivers/hwmon/f75375s.c: In function 'duty_mode_enabled':
drivers/hwmon/f75375s.c:280: warning: control reaches end of non-void function
drivers/hwmon/f75375s.c: In function 'auto_mode_enabled':
drivers/hwmon/f75375s.c:295: warning: control reaches end of non-void function

Fix the warning by returning something sensible after BUG().

Cc: Nikolaus Schulz <schulz@macnetix.de>
Cc: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>