regulator: hi6421v600: Fix .vsel_mask setting

Take ldo3_voltages as example, the ARRAY_SIZE(ldo3_voltages) is 16.
i.e. the valid selector is 0 ~ 0xF.
But in current code the vsel_mask is "(1 << 15) - 1", i.e. 0x7FFF. Fix it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Link: https://lore.kernel.org/r/20210529013236.373847-1-axel.lin@ingics.com
Signed-off-by: Mark Brown <broonie@kernel.org>