hwmon: (ina238) Simplify voltage register accesses

hwmon: (ina238) Simplify voltage register accesses

Calculate voltage LSB values in the probe function and use throughout
the code.

Use a single function to read all voltages, independently of the register
width. Use the pre-calculated LSB values to convert register values to
voltages and do not rely on runtime chip specific code.

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>