ASoC: samsung: i2s: Fix rclk_srcrate handling

As the RCLK clock may be updated through the common clk API before each
snd_soc_dai_ops::trigger call, it is not enough to update i2s->rclk_srcrate
only once after it has been initially set to 0. To avoid wrong PSR values
we always get RCLK frequency from the CLK_I2S_RCLK_SRC clock, when that
clock is available.

Fixes: e1417fdf3011 "ASoC: samsung: i2s: Ensure the RCLK rate is properly determined"
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>