hwmon: (mlxreg-fan) Modify PWM connectivity validation

hwmon: (mlxreg-fan) Modify PWM connectivity validation

Validate PWM connectivity only for additional PWM - "pwm1" is connected
on all systems, while "pwm2" - "pwm4" are optional. Validate
connectivity only for optional attributes by reading of related "pwm{n}"
registers - in case "pwm{n}" is not connected, register value is
supposed to be 0xff.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Link: https://lore.kernel.org/r/20210926053541.1806937-2-vadimp@nvidia.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>