mtd: sf: Drop plat from sf_probe

dm_spi_slave_platdata used in sf_probe for printing
plat->cs value and there is no relevant usage apart
from this.

We have enough debug messages available in SPI and SF
areas so drop this plat get and associated bug statement.

Cc: Simon Glass <sjg@chromium.org>
Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>