ASoC: bcm2835: Enforce full symmetry

bcm2835's configuration registers can't be changed when a stream
is running, which means asymmetric configurations aren't supported.

Channel and rate symmetry are already enforced by constraints
but samplebits had been missed.

As hw_params doesn't check for symmetry constraints by itself
and just returns success if a stream is running this led to
situations where asymmetric configurations were seeming to
succeed but of course didn't work because the hardware wasn't
configured at all.

Fix this by adding the missing samplerate symmetry constraint.

Signed-off-by: Matthias Reichl <hias@horus.com>