hwmon: (ina238) Add support for current limits

hwmon: (ina238) Add support for current limits

Since the shunt voltage register and the current register now report the
same values, use the shunt voltage limit registers to report and adjust
current limits, using the same LSB as the LSB used for the actual current
register.

Handle current register accuracy differences in separate function to
improve code readability.

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>