ASoC: samsung: i2s: Ensure the RCLK rate is properly determined

If the RCLK mux clock configuration is specified in DT and no set_sysclk()
callback is used in the sound card driver the sclk_srcrate field will remain
set to 0, leading to an incorrect PSR divider setting.
To fix this the frequency value is retrieved from the CLK_I2S_RCLK_SRC clock,
so the actual RCLK mux selection is taken into account.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>