spi/pxa2xx: initialize DMA channels to -1 to prevent inadvertent match

Commit cddb339b (spi/pxa2xx: convert to dma_request_slave_channel_compat())
converted the driver to use ACPI provided DMA helpers but it forgot to
initialize the platform data for the channels to -1. Failing to do so will
result inadvertent match in the filter function because 0 is a valid
channel number.

Prevent this from happening by initializing both platform data channels
correctly to -1.

Fixes: cddb339b (spi/pxa2xx: convert to dma_request_slave_channel_compat())
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org