ASoC: dwc: Defer bclk_ratio handling to hw_params

bclk_ratio is only a factor in clock producer mode, and needs to
override the default value of num_channels * sample_size.
Move the bclk_ratio handling into the hw_params method, only latching
the value in set_bclk_ratio, to address both of those matters.

See: https://github.com/raspberrypi/linux/issues/5817



Signed-off-by: Phil Elwell <phil@raspberrypi.com>