spi: fsl-espi: remove unneeded variable in fsl_espi_do_trans

Creating a message, adding one transfer, and then iterating over
all transfers in the message doesn't make sense.
We can simply use the original transfer directly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>