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: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
diff --git a/drivers/spi/ftssp010_spi.c b/drivers/spi/ftssp010_spi.c
index 267e4d8..c7d6480 100644
--- a/drivers/spi/ftssp010_spi.c
+++ b/drivers/spi/ftssp010_spi.c
@@ -431,7 +431,9 @@
 
 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)