staging: r8188eu: use proper way to build a module

staging: r8188eu: use proper way to build a module

It seems that for now, we can only build this driver as a module.

Use the same mechanism as other drivers (such as rtl8723bs or the
deprecated rtl8188eu) to enforce building as a module, i.e. depend on m
in Kconfig instead of setting CONFIG_R8188EU = m in the Makefile.

If we set CONFIG_R8188EU in the Makefile, this setting will not be visible
in .config.

Acked-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210811201450.31366-4-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>