spi: fsl-espi: factor out handling of read data

Factor out copying read data to the read buffers in the original
message to a new function fsl_espi_copy_from_buf.
This also allows to simplify fsl_espi_copy_to_buf.

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