mfd: ssbi: Constify buffer in ssbi_write

In preparation for passing a const pointer directly to
ssbi_write() from the regmap APIs.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
diff --git a/include/linux/ssbi.h b/include/linux/ssbi.h
index a92561a..bcbb642 100644
--- a/include/linux/ssbi.h
+++ b/include/linux/ssbi.h
@@ -17,7 +17,7 @@
 
 #include <linux/types.h>
 
-int ssbi_write(struct device *dev, u16 addr, u8 *buf, int len);
+int ssbi_write(struct device *dev, u16 addr, const u8 *buf, int len);
 int ssbi_read(struct device *dev, u16 addr, u8 *buf, int len);
 
 #endif