staging: wfx: fix support for BSS_CHANGED_KEEP_ALIVE

staging: wfx: fix support for BSS_CHANGED_KEEP_ALIVE

Chip firmware is able to send periodic null frames to keep the
association with the AP.

The driver arbitrary set this period to 30sec. We prefer to rely on
BSS_CHANGED_KEEP_ALIVE that provide a true value.

Note that if BSS_CHANGED_KEEP_ALIVE is not received, we just disable
keep_alive feature. It is not very disturbing since AP will probably
ping the station before to disconnect it.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200410133239.438347-9-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>