hwmon: (bt1-pvt) Mark is_visible functions static

0-day reports:

drivers/hwmon/bt1-pvt.c:303:16: warning:
	no previous declaration for 'pvt_limit_is_visible'
drivers/hwmon/bt1-pvt.c:308:16: warning:
	no previous declaration for 'pvt_alarm_is_visible'

Declare both functions static.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 87976ce2("hwmon: Add Baikal-T1 PVT sensor driver")
Cc: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>