hwmon: (pcf8591) Don't attempt to detect devices

The PCF8591 can't be detected, don't even try. There are plenty of
other means to instantiate i2c devices these days.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Reviewed-by: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>