ipm: correct prototype for ipm_send

ipm_send_t properly had 'const void *' for the data payload, but
not the ipm_send() API.

Change-Id: I62a424d37bc17f5bc3646c71d3ff3b029f52cbcc
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/include/ipm.h b/include/ipm.h
index 169ae77..83959b5 100644
--- a/include/ipm.h
+++ b/include/ipm.h
@@ -99,7 +99,7 @@
  * @retval 0        On success.
  */
 static inline int ipm_send(struct device *ipmdev, int wait, uint32_t id,
-			   void *data, int size)
+			   const void *data, int size)
 {
 	struct ipm_driver_api *api;