staging: wfx: always enable FastPs in combo with new firmwares

When multiple interface on different channels are in use. It is
necessary to advertise the AP that the device is no more awake before to
switch to the other channel.

Until now, PS-Poll was the preferred mechanism for that. However.  The
new firmwares (>= 3.7) now nicely support FastPS.

FastPS improves bandwidth and compatibility with AP.

This patch drop the complex and rarely used mechanism introduced in the
commit dd5eba1b ("staging: wfx: fix support for AP that do not
support PS-Poll") and use FastPS as soon as it is possible.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200701150707.222985-14-Jerome.Pouiller@silabs.com


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