spi: tegra20_sflash: Remove redundant code to set bus and cs of struct spi_slave

It's done in spi_alloc_slave(), thus remove the redundant code.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>