staging: rtl8188eu: force driver to be built as a module

The rtl8188eu driver defines a ton of global symbols which tend to
conflict with other realtek wifi drivers, force it to be built as
a module.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>