regulator: axp20x: Simplify axp20x_is_polyphase_slave implementation

The code to handle AXP803_ID and AXP813_ID cases are exactly the same.
Make the switch-case fall through to avoid duplicate code.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>