hwmon: (nct7904) Strengthen detect function

The bank register has five unused bits. Verify that those bits are zero
to strengthen the detect function.

Cc: Vadim V. Vlasov <vvlasov@dev.rtsoft.ru>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>