spi: dw-mid: refactor to use helpers

commit a5c2db964d3eb26b41bd7abc1b13486f732b3aa2 upstream.

This patch splits few helpers, namely dw_spi_dma_prepare_rx(),
dw_spi_dma_prepare_tx(), and dw_spi_dma_setup() which will be useful for the
consequent improvements.

There is no functional change.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
[removes a build warning with newer versions of gcc - gregkh]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>