Remove a write-only variable, spotted by GCC 4.6.0:
/src/qemu/hw/petalogix_ml605_mmu.c: In function 'petalogix_ml605_init':
/src/qemu/hw/petalogix_ml605_mmu.c:153:11: error: variable 'serial' set but not used [-Werror=unused-but-set-variable]
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>