Fix SkFixedDiv macro

Previously, the macro won't pass the unit test that I just added in this
CL.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2149733002

Review-Url: https://codereview.chromium.org/2149733002
diff --git a/tests/MathTest.cpp b/tests/MathTest.cpp
index 23785f7..1dd8223 100644
--- a/tests/MathTest.cpp
+++ b/tests/MathTest.cpp
@@ -530,6 +530,8 @@
         REPORTER_ASSERT(reporter, result == SK_Fixed1);
         result = SkFixedDiv(1, SK_Fixed1);
         REPORTER_ASSERT(reporter, result == 1);
+        result = SkFixedDiv(10 - 1, SK_Fixed1 * 3);
+        REPORTER_ASSERT(reporter, result == 3);
     }
 
     unittest_fastfloat(reporter);