firmware: qemu_fw_cfg.c: potential unintialized variable

It acpi_acquire_global_lock() return AE_NOT_CONFIGURED then "glk" isn't
initialized, which, if you got very unlucky, could cause a bug.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>