staging: wfx: add a debugfs entry to force ps_timeout

staging: wfx: add a debugfs entry to force ps_timeout

In some advanced usage or debug scenarios, it could interesting to
change the value of ps_timeout or eventually to force use of PS-Poll
frames.

The wext API (used by iwconfig) provide a way to change ps_timeout.
However, this API is obsolete and it seems a little weird to use (it
seems it does apply the change, so the user have to disable then
re-enable de power save)

On side of nl80211, there is no way to change the ps_timeout.

This patch provides a file in debugfs to change the value of ps_timeout.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200701150707.222985-13-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>