dmaengine: dw: Fix FIFO size for Intel Merrifield

Intel Merrifield has a reduced size of FIFO used in iDMA 32-bit controller,
i.e. 512 bytes instead of 1024.

Fix this by partitioning it as 64 bytes per channel.

Note, in the future we might switch to 'fifo-size' property instead of
hard coded value.

Fixes: 199244d6

 ("dmaengine: dw: add support of iDMA 32-bit hardware")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>