si: Make sure to not overwrite orig_arfcn_hi in the range encoding

Andreas highlighted that the doubel assignment is not needed and
wrong. Change the code to assign chan_list[0] before writing the
base frequency to the header. Update the testcase to make the highest
bit set and update the test result.
diff --git a/openbsc/tests/si/si_test.c b/openbsc/tests/si/si_test.c
index 16f6ea3..fd840f3 100644
--- a/openbsc/tests/si/si_test.c
+++ b/openbsc/tests/si/si_test.c
@@ -103,7 +103,7 @@
 			break;
 		}
 
-	rc = range_enc_range512(chan_list, 0x96, w);
+	rc = range_enc_range512(chan_list, (1 << 9) | 0x96, w);
 	VERIFY(rc, ==, 0);
 
 	printf("Range512: %s\n", osmo_hexdump(chan_list, ARRAY_SIZE(chan_list)));