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/core.c b/core.c
index 6e59993..8a1796f 100644
--- a/core.c
+++ b/core.c
@@ -105,7 +105,7 @@
 	return (void *)pkt->data + pkt->mctp_hdr_off + sizeof(struct mctp_hdr);
 }
 
-uint8_t mctp_pktbuf_size(struct mctp_pktbuf *pkt)
+size_t mctp_pktbuf_size(struct mctp_pktbuf *pkt)
 {
 	return pkt->end - pkt->start;
 }