hwmon: (f75375s) Disable setting DC fan control mode for F75373

F75373 does not support DC (linear voltage) fan control mode, so don't let
the user set it.

Cc: Riku Voipio <riku.voipio@iki.fi>
Reviewed-by: Bjoern Gerhart <oss@extracloud.de>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>