schema: Accept band GSM-900

Change-Id: I274c3010f33776b2967ee01de027c44b794cc5d4
diff --git a/src/osmo_gsm_tester/schema.py b/src/osmo_gsm_tester/schema.py
index 72649cc..487521a 100644
--- a/src/osmo_gsm_tester/schema.py
+++ b/src/osmo_gsm_tester/schema.py
@@ -39,7 +39,7 @@
     raise ValueError('Invalid %s: %r' % (name, val))
 
 def band(val):
-    if val in ('GSM-1800', 'GSM-1900'):
+    if val in ('GSM-900', 'GSM-1800', 'GSM-1900'):
         return
     raise ValueError('Unknown GSM band: %r' % val)