bl.minor: avoid unsigned comparison warning

Comparing bl.minor to LEVEL_MINOR_MIN gives a warning because
bl.minor is unsigned and LEVEL_MINOR_MIN is 0.

Change-Id: I15ebb2e90191d681f131df19bdfde1625ae1db58
diff --git a/av1/common/onyxc_int.h b/av1/common/onyxc_int.h
index e684521..64d36ad 100644
--- a/av1/common/onyxc_int.h
+++ b/av1/common/onyxc_int.h
@@ -1334,7 +1334,10 @@
 
 static INLINE uint8_t major_minor_to_seq_level_idx(BitstreamLevel bl) {
   assert(bl.major >= LEVEL_MAJOR_MIN && bl.major <= LEVEL_MAJOR_MAX);
-  assert(bl.minor >= LEVEL_MINOR_MIN && bl.minor <= LEVEL_MINOR_MAX);
+  // Since bl.minor is unsigned a comparison will return a warning:
+  // comparison is always true due to limited range of data type
+  assert(LEVEL_MINOR_MIN == 0);
+  assert(bl.minor <= LEVEL_MINOR_MAX);
   return ((bl.major - LEVEL_MAJOR_MIN) << LEVEL_MINOR_BITS) + bl.minor;
 }