regulator: lp87565: Use bitfield helpers

Use the FIELD_PREP() helper, instead open-coding the same operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://msgid.link/r/5e8781d31c983caf6bdabe36c5552077422cb8da.1708002575.git.geert+renesas@glider.be


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