tcg: Fix compiler error (comparison of unsigned expression)

When qemu is configured with --enable-debug-tcg,
gcc throws this warning (or error with -Werror):

tcg/tcg.c:1030: error: comparison of unsigned expression >= 0 is always true

Fix it by removing the >= 0 part.
The type cast to 'unsigned' catches negative values of op
(which should never happen).

This is a modification of Hollis Blanchard's patch.

Cc: Hollis Blanchard <hollis@penguinppc.org>
Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

tcg/tcg.c

@@ -1027,7 +1027,7 @@ void tcg_add_target_add_op_defs(const TCGTargetOpDef *tdefs)
         if (tdefs->op == (TCGOpcode)-1)
             break;
         op = tdefs->op;
-        assert(op >= 0 && op < NB_OPS);
+        assert((unsigned)op < NB_OPS);
         def = &tcg_op_defs[op];
 #if defined(CONFIG_DEBUG_TCG)
         /* Duplicate entry in op definitions? */