spi: imx: use pio mode for i.mx6dl

commit a02bb401f8ae264be782ee57d98bdd99f14c8022 upstream.

For TKT238285 hardware issue which may cause txfifo store data twice can only
be caught on i.mx6dl, we use pio mode instead of DMA mode on i.mx6dl.

Fixes: f62caccd (spi: spi-imx: add DMA support)
Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>