spi: cf_spi: Use to_cf_spi_slave to resolve cfslave from slave

Don't assume slave is always the first member of struct cf_spi_slave.
Use container_of instead of casting first structure member.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>