spi: spi-fsl-dspi: Use max_native_cs instead of num_chipselect to set SPI_MCR

If cs-gpios property is used in devicetree then ctlr->num_chipselect value
may be changed by spi_get_gpio_descs().
So use ctlr->max_native_cs instead of ctlr->num_chipselect to set SPI_MCR

Fixes: 4fcc7c22

 (spi: spi-fsl-dspi: Don't access reserved fields in SPI_MCR)
Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Link: https://lore.kernel.org/r/20201201085916.63543-1-fido_max@inbox.ru
Signed-off-by: Mark Brown <broonie@kernel.org>