staging: wfx: update power-save per interface

mac80211 and the device are both able to control power-save per vif.
But, the current code retrieve power-save from wfx_config(). So, it does
not allow to setup power-save independently for each vif. Driver just
has to rely on wfx_bss_info_changed() instead of wfx_config().

wfx_config() has nothing to do anymore, but we keep it since it is
mandatory for mac80211.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200115135338.14374-41-Jerome.Pouiller@silabs.com


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