hwmon: (f71882fg) Mark expected switch fall-through

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

This patch fixes the following warnings:

drivers/hwmon/f71882fg.c: In function ‘f71882fg_probe’:
drivers/hwmon/f71882fg.c:2457:33: warning: this statement may fall through [-Wimplicit-fallthrough=]
    data->auto_point_temp_signed = 1;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
drivers/hwmon/f71882fg.c:2459:3: note: here
   case f71889fg:
   ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>