staging: wfx: relocate "buffered" information to sta_priv

It simplify the code if field buffered is hosted in the struct sta_priv
instead of in the struct wfx_link_entry. More globally, struct
wfx_link_entry has no real reasons to exist and should be dropped soon.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200115135338.14374-47-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>