ASoC: kirkwood-dma: remove channel restrictions

This is part of a patch found in Rabeeh Khoury's git tree for the
cubox.

With SPDIF passthrough, we are not restricted to just two channels of
audio; we can support however many channels the non-audio stream can
itself support.  In any case, kirkwood-dma is not involved in the
format selection.  So yet rid of this restriction.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>