staging: wfx: fix support for AP that do not support PS-Poll

staging: wfx: fix support for AP that do not support PS-Poll

When multiple virtual interfaces (on different channels) are in use, the
device ask to activate Power Save on station interfaces. The device
developers recommends to use legacy PS-Poll in this case since it is the
mode that disturb the less the other interface. However, some AP start
to not answer anymore to PS-Poll. The device is able to detect this case
and return a special warning in this case.

So, this commit catch the warning and force usage of FastPS in this
case.

In order to confuse the less possible the other interface a small FastPS
period is used (30ms).

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200427134031.323403-5-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>