static-checks: use origin/master instead of master as base branch

In some CI testing scenarios, there are no local branches, just remotes, so
as a safer approach, use 'origin/master' instead of 'master'. Base
branch is used to find common parent ancestor between
base (origin/master) and user patchset's (FETCH_HEAD) branches.

Change-Id: I36940ac043f45bda6b94e8787b29f319a5f4acba
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
diff --git a/script/static-checks/check-banned-api.py b/script/static-checks/check-banned-api.py
index 0bfadeb..931b1bd 100755
--- a/script/static-checks/check-banned-api.py
+++ b/script/static-checks/check-banned-api.py
@@ -168,7 +168,7 @@
                         help="""
                         Base commit in patch mode (default: %(default)s)
                         """,
-                        default="master")
+                        default="origin/master")
     parser.add_argument("--to-ref",
                         help="""
                         Final commit in patch mode (default: %(default)s)