ASoC: rsnd: Add missing initialization of ADG req_rate

If the "clock-frequency" DT property is not found, req_rate is used
uninitialized, and the "audio_clkout" clock will be created with an
arbitrary clock rate.

This uninitialized kernel stack data may leak to userspace through
/sys/kernel/debug/clk/clk_summary, cfr. the value in the "rate" column:

       clock     enable_cnt  prepare_cnt        rate   accuracy   phase
    --------------------------------------------------------------------
     audio_clkout         0            0  4001836240          0 0

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>