spi: imx: fix issue when tx_buf or rx_buf is NULL

In case either transfer->tx_buf or transfer->rx_buf is NULL,
manipulation of buffer in spi_imx_u32_swap_u[8|16]() will cause
NULL pointer dereference crash.

Add buffer check at very beginning of spi_imx_u32_swap_u[8|16](),
to avoid such crash.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Reported-by: Leonard Crestez <leonard.crestez@nxp.com>
Tested-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>