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>