It makes no sense to attempt to manually configure the fan in auto mode,
or set the duty cycle directly in closed loop mode.  The corresponding
registers are then read-only.  If the user tries it nonetheless, error out
with EINVAL instead of silently doing nothing.
Signed-off-by: Nikolaus Schulz <mail@microschulz.de>
[guenter.roeck@ericsson.com: Minor formatting cleanup]
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>