hwmon: (pmbus) Add support for reading direct mode coefficients

hwmon: (pmbus) Add support for reading direct mode coefficients

Add support for reading and decoding direct format coefficients to
the PMBus core driver. If the new flag PMBUS_USE_COEFFICIENTS_CMD
is set, the driver will use the COEFFICIENTS register together with
the information in the pmbus_sensor_attr structs to initialize
relevant coefficients for the direct mode format.

Signed-off-by: Erik Rosen <erik.rosen@metormote.com>
[groeck: Initialize ret with -EINVAL in pmbus_init_coefficients()]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>