spi: dw: Move dw_spi_can_dma()

Move dw_spi_can_dma() implementation below dw_spi_dma_convert_width()
for handing compile dependency in future patches.

* tested on Baikal-T1 based system with DW SPI-looped back interface
transferring a chunk of data with DFS:8,12,16.

Signed-off-by: Joy Chakraborty <joychakr@google.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20230427123314.1997152-3-joychakr@google.com


Signed-off-by: Mark Brown <broonie@kernel.org>