bsc_api: Change the cause to a uint8_t for now

The RR Cause can only be one byte. Change from uint16_t to uint8_t for that.
diff --git a/openbsc/src/bsc/osmo_bsc_api.c b/openbsc/src/bsc/osmo_bsc_api.c
index 7b9f778..f1ba559 100644
--- a/openbsc/src/bsc/osmo_bsc_api.c
+++ b/openbsc/src/bsc/osmo_bsc_api.c
@@ -44,7 +44,7 @@
 {
 }
 
-static void bsc_assign_compl(struct gsm_subscriber_connection *conn, uint16_t rr_cause)
+static void bsc_assign_compl(struct gsm_subscriber_connection *conn, uint8_t rr_cause)
 {
 }