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>

src/drivers/spi/eon.c

@@ -126,7 +126,7 @@ out:
 	return ret;
 }
 
-int spi_flash_probe_eon(struct spi_slave *spi, u8 *idcode,
+int spi_flash_probe_eon(const struct spi_slave *spi, u8 *idcode,
 			struct spi_flash *flash)
 {
 	const struct eon_spi_flash_params *params;