spi_qsd: Ensure the DMA callback is called before completion.

There may be some latency in the DMA IRQ firing in hardware.
Hence, in the case of write transfers, the QUP interrupt handler
waits for the DMA callback before issuing completion.

Change-Id: I4962c5c04c3a45dbf6e98165d2815adda657a9fb
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>