ASoC: atmel_ssc_dai: if not provided, default to sensible dividers

When this driver masters BCLK and/or LRCLK, and noone has stated
differently, assume that all the bits of a frame are used.

This relieves the cpu dai users from the duty to fill in the dividers for
the common case.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Mark Brown <broonie@kernel.org>