spi: imx: move wml setting to later than setup_transfer

Current dynamic burst length is based on the whole transfer length,
that's ok if there is only one sg, but is not right in case multi sgs
in one transfer,because the tail data should be based on the last sg
length instead of the whole transfer length. Move wml setting for DMA
to the later place, thus, the next patch could get the right last sg
length for wml setting. This patch is a preparation one, no any
function change involved.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>