spi: cf_qspi: Fixup to_cf_qspi_slave macro

The third parameter of container_of is the name of the member within the struct.
Current code only works if the parameter passed to to_cf_qspi_slave named slave.
Fix it.

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