spi: dw-mid: respect 8 bit mode

commit b41583e7299046abdc578c33f25ed83ee95b9b31 upstream.

In case of 8 bit mode and DMA usage we end up with every second byte written as
0. We have to respect bits_per_word settings what this patch actually does.

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