hwmon: (bt1-pvt) Cache current update timeout

hwmon: (bt1-pvt) Cache current update timeout

Instead of converting the update timeout data to the milliseconds each
time on the read procedure let's preserve the currently set timeout in the
dedicated driver private data cache. The cached value will be then used in
the timeout read method and in the alarm-less data conversion to prevent
the caller task hanging up in case if the PVT sensor is suddenly powered
down.

Fixes: 87976ce2825d ("hwmon: Add Baikal-T1 PVT sensor driver")
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20200920110924.19741-3-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Guenter Roeck <linux@roeck-us.net>