ASoC: bcm: Use power-of-2 bclk_ratios

The soundcard drivers originally used snd_pcm_format_physical_width,
but a later commit changed that to snd_pcm_format_width because the
in-memory sample storage width should not be a factor in determining
the bclk_ratio. However, the physical width rounds the sample bits up
to the nearest power of 2, which makes it easier to find integer clock
divisors.

Restore the old behaviour, but with an implementation that makes it
clear what is going on.

See: https://github.com/raspberrypi/linux/issues/6104



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