hwmon: (pmbus/mp2975) Make phase count variable

In order to add support for MP2973 and MP2971 replace hardcoded
phase count for both channels by a variable.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Signed-off-by: Naresh Solanki <Naresh.Solanki@9elements.com>
Link: https://lore.kernel.org/r/20230714135124.2645339-5-Naresh.Solanki@9elements.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>