spi: sifive: Tidy up dm_spi_slave_platdata variable

Usually variable name slave is used for spi_slave structure
and slave_plat for the dm_spi_slave_platdata.

Let's follow this meaningful notation by replacing slave
with slave_plat for dm_spi_slave_platdata structure.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>