drivers: spi: Set dummy data in mcux drivers

Sets the "dummy data" value to send when the transmit buffer is null.
Fixes the spi_null_tx_buf test in tests/drivers/spi/spi_loopback on the
lpcxpresso54114 board.

Tested on frdm_k64f, mimxrt1050_evk, and lpcxpresso54114_m4 boards.

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