net: pse-pd: tps23881: Use helpers to calculate bit offset for a channel

This driver frequently follows a pattern where two registers are read or
written in a single operation, followed by calculating the bit offset for
a specific channel.

Introduce helpers to streamline this process and reduce code redundancy,
making the codebase cleaner and more maintainable.

Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>