regulator: axp20x: Fix poly-phase bit offset for AXP803 DCDC5/6

The bit offset used to check if DCDC5 and DCDC6 are tied together in
poly-phase output is wrong. It was checking against a reserved bit,
which is always false.

In reality, neither the reference design layout nor actually produced
boards tie these two buck regulators together. But we should still
fix it, just in case.

Fixes: 1dbe0ccb ("regulator: axp20x-regulator: add support for AXP803")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Tested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>