staging: r8188eu: Makefile: use one file list

It's enough to have a single list of source files. There are no config
settings for this driver that change the list of source files that
we compile.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211016173544.25376-4-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>