spi: fsl_dspi fix to stop extra transmissions

Update to prevent a byte of zeros being transmitted between
each byte in the tx buffer when providing both a tx buffer
and a rx buffer.

Signed-off-by: Jared Bents <jared.bents@rockwellcollins.com>
Cc: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>