spi: dw: Add 'mfld' suffix to Intel Medfield related routines

In order to prepare driver for the extension to support newer hardware,
add 'mfld' suffix to some related functions.

While here, move DMA parameters assignment under existing #ifdef
CONFIG_SPI_DW_MID_DMA.

There is no functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200506153025.21441-6-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>