bq2415x_charger: Fix max battery regulation voltage

As per the datasheets, maximum battery regulation voltage is 4440mV.

The formula is (voltage - offset) / step, so the maximum value is:
(4440 - 3500) / 20 = 47

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Thanks-to: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>