spi: ftssp010_spi: Use to_ftssp010_spi() to ensure free correct address

Don't assume slave is always the first member of struct ftssp010_spi.
Use to_ftssp010_spi() to ensure free correct address in spi_free_slave().
Signed-off-by: NAxel Lin <axel.lin@ingics.com>
Reviewed-by: NJagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

drivers/spi/ftssp010_spi.c

@@ -431,7 +431,9 @@ free_out:
 
 void spi_free_slave(struct spi_slave *slave)
 {
-	free(slave);
+	struct ftssp010_spi *chip = to_ftssp010_spi(slave);
+
+	free(chip);
 }
 
 int spi_claim_bus(struct spi_slave *slave)