hwmon: (corsair-cpro) add reading pwm values

This adds the possibility for reading pwm values.
These can not be read if the device is controlled via
fan_target or a fan curve and will return an error in
this case. Since an error is expected, this adds some
rudimentary error handling.

Changes:
- add CTL_GET_FAN_PWM and use it via get_data
- pwm returns -ENODATA if the device returns error 0x12
- fan_target now returns -ENODATA when the driver is
  started or a pwm value is set.
- add ccp_get_errno to determine errno from device error.
- get_data now has a parameter to determine whether
  to read one or two bytes of data.
- update documentation
- fix missing surname in MAINTAINERS

Signed-off-by: Marius Zachmann <mail@mariuszachmann.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>