hwmon: (mr75203) fix multi-channel voltage reading

Fix voltage allocation and reading to support all channels in all VMs.
Prior to this change allocation and reading were done only for the first
channel in each VM.
This change counts the total number of channels for allocation, and takes
into account the channel offset when reading the sample data register.

Fixes: 9d823351

 ("hwmon: Add hardware monitoring driver for Moortec MR75203 PVT controller")
Signed-off-by: Eliav Farber <farbere@amazon.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220908152449.35457-6-farbere@amazon.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>