Drop ret variable from vc3_clk_mux_determine_rate().

While at it, return the value returned by regmap_*
wherever possible instead of returning 0.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20231122142310.203169-6-biju.das.jz@bp.renesas.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>