drivers/spi/spi_flash: Pass in spi_slave structure as const to probe functions

Pointer to spi_slave structure can be passed in as const to spi flash
probe functions since the probe functions do not need to modify the
slave properties.

BUG=b:38330715

Change-Id: I956ee777c62dbb811fd6ce2aeb6ae090e1892acd
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19707
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
16 files changed