drivers: spi: Set spi context for mcux flexcomm spi slave configuration

This is a bug fix. A pointer to the spi configuration is not saved when
the spi driver is configured for slave operation and it can lead to
runtime errors.

Signed-off-by: Bryce Wilkins <bryce.wilkins@gmail.com>

drivers/spi/spi_mcux_flexcomm.c

@@ -289,6 +289,8 @@ static int spi_mcux_configure(const struct device *dev,
 
 		SPI_SlaveTransferCreateHandle(base, &data->handle,
 					      spi_mcux_transfer_callback, data);
+
+		data->ctx.config = spi_cfg;
 	}
 
 	return 0;