spi/imx: do not make copy of spi_imx_devtype_data

spi_imx_devtype_data has already been driver private data.  There is
really no need to make a copy in spi_imx_data.  Instead, a reference
pointer works perfectly fine.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>