resource: ipa_unit_id is expected to be a positive integer

Change-Id: I26d7a4351089b73716d193496d7161bf3d9988bb
diff --git a/src/osmo_gsm_tester/resource.py b/src/osmo_gsm_tester/resource.py
index da543f7..f8acb9e 100644
--- a/src/osmo_gsm_tester/resource.py
+++ b/src/osmo_gsm_tester/resource.py
@@ -51,7 +51,7 @@
         'ip_address[].addr': schema.IPV4,
         'bts[].label': schema.STR,
         'bts[].type': schema.STR,
-        'bts[].ipa_unit_id': schema.INT,
+        'bts[].ipa_unit_id': schema.UINT,
         'bts[].addr': schema.IPV4,
         'bts[].band': schema.BAND,
         'bts[].trx_remote_ip': schema.IPV4,