hwmon: (ina238) Pre-calculate current, power, and energy LSB

hwmon: (ina238) Pre-calculate current, power, and energy LSB

Current, power, and energy LSB do not change during runtime, so we can
pre-calculate the respective values. The power LSB can be derived from
the current LSB using the equation in the datasheets. Similar, the
energy LSB can be derived from the power LSB.

Also add support for chips with built-in shunt resistor by providing
a chip specific configuration parameter for the current LSB. The
relationship of current -> power -> energy LSB values in those chips
is the same as in chips with external shunt resistor, so configuration
parameters for power and energy LSB are not needed.

Use ROUND_CLOSEST functions instead of divide operations to reduce
rounding errors.

Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz> # INA780
Signed-off-by: Guenter Roeck <linux@roeck-us.net>