ASoC: kirkwood-dma: remove restriction on sample rates

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

The kirkwood DMA hardware for ASoC does not impose any restrictions
on the sample rates available, so it's silly to impose an artificial
set in the DMA code.  The restrictions come from the availble clocks
to the I2S module, which are already handled in the I2S part of the
driver.

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