drivers: spi: mcux_flexcomm: fix DMA bug for 2-byte transfers

The MCUX DMA controller only supports a single data_size
for a DMA transfer, not separate ones for source and
dest. An older version of the DMA driver used
dest_data_size as the DMA transfer size, but the
current one uses MIN(dest/source) as the trasnfer
size, which breaks case when SPI wants to do 2-byte
transfers.

Signed-off-by: Mike J. Chen <mjchen@google.com>