hwmon: (adt7475) Handle alternative pin functions

The TACH4 pin can be used for other functions, so fan4 may not always
be available. Likewise, the PWM2 pin can be used for ALERT output, in
which case pwm2 is not available

For the ADT7490, the +2.5 Vin pin may also be used for other
functions, in which case in0 is not available.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Jordan Crouse <jordan@cosmicpenguin.net>
Cc: "Darrick J. Wong" <djwong@us.ibm.com>