mfd: bd71828: Fix .n_voltages settings

Current .n_voltages settings do not cover the latest 2 valid selectors,
so it fails to set voltage for the hightest voltage support.
The latest linear range has step_uV = 0, so it does not matter if we
count the .n_voltages to maximum selector + 1 or the first selector of
latest linear range + 1.
To simplify calculating the n_voltages, let's just set the
.n_voltages to maximum selector + 1.

Fixes: 522498f8 ("regulator: bd71828: Basic support for ROHM bd71828 PMIC regulators")
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>