ASoC: bcm: Use the correct sample width value

ALSA's concept of the physical width of a sample is how much memory it
occupies, including any padding. This not the same as the count of bits
of actual sample content. In particular, S24_LE has a width of 24 bits
but a physical width of 32 bits because there is a byte of padding with
each sample.

When calculating bclk_ratio, etc., it is width that matters, not
physical width. Correct the error that has been replicated across the
drivers for many Raspberry Pi-compatible soundcards.

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