mips: avoid write only variables

Compiling with GCC 4.6.0 20100925 produced a lot of warnings like:
/src/qemu/target-mips/translate.c: In function 'gen_ld':
/src/qemu/target-mips/translate.c:1039:17: error: variable 'opn' set but not used [-Werror=unused-but-set-variable]

Fix by adding a dummy cast so that the variable is not unused.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>