spi: fsl-espi: remove remaining usage of struct fsl_spi_platform_data

Use master->num_chipselect directly instead of pdata->max_chipselect.
In this context let of_fsl_espi_get_chipselects return max_chipselect.

This change allows us to get rid of struct fsl_spi_platform_data
completely in the fsl-espi driver.

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