spi: fsl-espi: eliminate struct fsl_espi_transfer

The remaining elements of struct fsl_espi_transfer are part of struct
spi_transfer anyway. So we can get rid of struct fsl_espi_transfer
and use a struct spi_transfer only.

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