Authentication: use ENUM instead of magic numbers

This improves readability of the code...
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index c3bf64f..02854b6 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -191,10 +191,10 @@
 		/* FIXME: Should start a timer for completion ... */
 
 	/* Then do whatever is needed ... */
-	if (rc == 1) {
+	if (rc == AUTH_DO_AUTH_THAN_CIPH) {
 		/* Start authentication */
 		return gsm48_tx_mm_auth_req(conn, op->atuple.rand, op->atuple.key_seq);
-	} else if (rc == 2) {
+	} else if (rc == AUTH_DO_CIPH) {
 		/* Start ciphering directly */
 		return gsm0808_cipher_mode(conn, net->a5_encryption,
 		                           op->atuple.kc, 8, 0);