drivers: dai: intel: dmic: don't use assert for error handling

The dai.h interface does not prohibit calling dai_config_get()
with different direction values. The dmic driver should handle
invalid direction value explicitly and not rely on an assert.

Link: https://github.com/thesofproject/sof/issues/6896


Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>