smbios: Fix dimm size calculation when RAM is multiple of 16GB

smbios: Fix dimm size calculation when RAM is multiple of 16GB

The Memory Device size calculation logic is broken when the RAM size is
a multiple of 16GB, making the size of the last entry be 0 instead of
16GB. Fix the logic to handle that case correctly.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>