spi: dw-mid: follow new DMAengine workflow

Though intel-mid-dma does not follow a new DMA workflow (*) let's prepare SPI
DW driver for that.

(*) The client is obliged to call dma_async_issue_pending() which starts the
actual transfers. Old DMA drivers do not follow this, since requirement was
introduced in the discussion of [1].

[1] http://www.spinics.net/lists/arm-kernel/msg125987.html

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>