staging: rtl8723bs: force driver to be built as a module.

Due to the ton of global symbols, the driver has to be a module
otherwise the build blows up if it is compiled into the kernel.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>