hwmon: (f71882fg) Fix fan_to/from_reg prototypes

The RPM after conversion from / before conversion to a register value can be
much more than 65535 (up to 1500000), so putting this into an u16 can cause
overflows. This changes the functions to use an int to store / get RPM instead.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>