mboxd_msg: Make raw bytes a uint8_t for sanity

Remove the ambiguity of signed/unsigned char.

Change-Id: I979d1907f7be08a9413770839909a30f4640701a
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/mboxd_msg.h b/mboxd_msg.h
index 6ef6b27..94ac13a 100644
--- a/mboxd_msg.h
+++ b/mboxd_msg.h
@@ -20,7 +20,7 @@
 };
 
 union mbox_regs {
-	char raw[MBOX_REG_BYTES];
+	uint8_t raw[MBOX_REG_BYTES];
 	struct mbox_msg msg;
 };