clk: meson: audio-divider is one based

The audio divider is one based. This offset was mistakenly dropped from
recalc_rate() when migrating to clk_regmap.

Fixes: 88a4e128 ("clk: meson: migrate the audio divider clock to clk_regmap")
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>