spi: Fix mcux dspi driver to parse lsb transfer mode correctly

The mcux dspi driver was incorrectly using the macro SPI_MODE_GET() to
parse the operation for SPI_TRANSFER_LSB. The effect of this bug was
that the driver would quietly always operate in SPI_TRANSFER_MSB mode.

Coverity-CID: 185401

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>