Define the max register address the device supports.
This allows reading the whole register space via
regmap debugfs, without it only register 0x0 is visible.

This was forgotten in the original driver commit.

Fixes: fff7b8ab ("hwmon: add Texas Instruments TPS23861 driver")
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Link: https://lore.kernel.org/r/20210609220728.499879-1-robert.marko@sartura.hrSigned-off-by: Guenter Roeck <linux@roeck-us.net>