staging: wfx: explain uncommon Makefile statement

I have got questions about this line from several reviewers. A comment
is definitively welcome.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20220113085524.1110708-6-Jerome.Pouiller@silabs.com


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