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: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
