core: Address a handful of compiler warnings

Address a handful of compiler warnings where void pointers are
incremented.

Suggested-by: Younghyun Park <younghyunpark@google.com>
Signed-off-by: Moritz Fischer <moritzf@google.com>
Change-Id: I6171efce851a6b9585b069d12daa389e2338a98b
diff --git a/core.c b/core.c
index c3ee659..e878175 100644
--- a/core.c
+++ b/core.c
@@ -117,12 +117,12 @@
 
 struct mctp_hdr *mctp_pktbuf_hdr(struct mctp_pktbuf *pkt)
 {
-	return (void *)pkt->data + pkt->mctp_hdr_off;
+	return (struct mctp_hdr *)(pkt->data + pkt->mctp_hdr_off);
 }
 
 void *mctp_pktbuf_data(struct mctp_pktbuf *pkt)
 {
-	return (void *)pkt->data + pkt->mctp_hdr_off + sizeof(struct mctp_hdr);
+	return pkt->data + pkt->mctp_hdr_off + sizeof(struct mctp_hdr);
 }
 
 size_t mctp_pktbuf_size(struct mctp_pktbuf *pkt)
@@ -261,7 +261,7 @@
 		}
 	}
 
-	memcpy(ctx->buf + ctx->buf_size, mctp_pktbuf_data(pkt), len);
+	memcpy((uint8_t *)ctx->buf + ctx->buf_size, mctp_pktbuf_data(pkt), len);
 	ctx->buf_size += len;
 
 	return 0;
@@ -793,7 +793,7 @@
 		hdr->flags_seq_tag |=
 			(i & MCTP_HDR_SEQ_MASK) << MCTP_HDR_SEQ_SHIFT;
 
-		memcpy(mctp_pktbuf_data(pkt), msg + p, payload_len);
+		memcpy(mctp_pktbuf_data(pkt), (uint8_t *)msg + p, payload_len);
 
 		/* add to tx queue */
 		if (bus->tx_queue_tail)