spi: fsl-dspi: Store status directly in cur_msg->status

This will allow us to return a status from the interrupt handler in a
later commit and avoids copying it at the end of
dspi_transfer_one_message(). For consistency make polling and DMA modes
use the same mechanism.

No functional changes intended.

Signed-off-by: James Clark <james.clark@linaro.org>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Message-ID: <20250902-james-nxp-spi-dma-v6-2-f7aa2c5e56e2@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>