Add 'const' keyword to chip write and other function prototypes

Corresponding to flashrom svn r1789.

Inspired by and mostly based on a patch
Signed-off-by: Mark Marshall <mark.marshall@omicron.at>

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
diff --git a/ichspi.c b/ichspi.c
index 6c394db..190ad5a 100644
--- a/ichspi.c
+++ b/ichspi.c
@@ -1327,8 +1327,7 @@
 	return 0;
 }
 
-static int ich_hwseq_write(struct flashctx *flash, uint8_t *buf,
-			   unsigned int addr, unsigned int len)
+static int ich_hwseq_write(struct flashctx *flash, const uint8_t *buf, unsigned int addr, unsigned int len)
 {
 	uint16_t hsfc;
 	uint16_t timeout = 100 * 60;