core: Return size_t for mctp_pktbuf_size()

Large packets are enabled by e.g. MTU negotiation in the astlpc binding.
Avoid truncating large packets in calls to the pktbuf APIs.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I1244d70fe93ca7538ca09256110d648c495c6aa7
diff --git a/libmctp.h b/libmctp.h
index 40e5371..a8beb72 100644
--- a/libmctp.h
+++ b/libmctp.h
@@ -55,7 +55,7 @@
 void mctp_pktbuf_free(struct mctp_pktbuf *pkt);
 struct mctp_hdr *mctp_pktbuf_hdr(struct mctp_pktbuf *pkt);
 void *mctp_pktbuf_data(struct mctp_pktbuf *pkt);
-uint8_t mctp_pktbuf_size(struct mctp_pktbuf *pkt);
+size_t mctp_pktbuf_size(struct mctp_pktbuf *pkt);
 void *mctp_pktbuf_alloc_start(struct mctp_pktbuf *pkt, size_t size);
 void *mctp_pktbuf_alloc_end(struct mctp_pktbuf *pkt, size_t size);
 int mctp_pktbuf_push(struct mctp_pktbuf *pkt, void *data, size_t len);