spi_qsd: Do not fail probe on DMA init error

Rather than failing the probe function on DMA
initialiation error, this patch just disables DMA
mode and logs an error. The driver continues to
function in FIFO mode.

Change-Id: Ib2532e3497192e1ef7efa02d2dff855826ad6091
Signed-off-by: Gilad Avidov <gavidov@codeaurora.org>