regulator: tps6287x: Fix n_voltages

There are 256 possible voltage settings for each range, not 256 possible
voltage settings in total.

Fixes: 15a1cd24 ("regulator: tps6287x: Fix missing .n_voltages setting")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20230829-tps-voltages-v1-1-7ba4f958a194@axis.com


Signed-off-by: Mark Brown <broonie@kernel.org>