sercomm: make rx msg size configurable per instance

Change-Id: I835506e26e83232e1e7290c0da04d389c8d7fb40
diff --git a/include/osmocom/core/sercomm.h b/include/osmocom/core/sercomm.h
index 10deb7c..d84786e 100644
--- a/include/osmocom/core/sercomm.h
+++ b/include/osmocom/core/sercomm.h
@@ -39,6 +39,7 @@
 	/* receive side */
 	struct {
 		dlci_cb_t dlci_handler[_SC_DLCI_MAX];
+		unsigned int msg_size;
 		struct msgb *msg;
 		int state;
 		uint8_t dlci;