staging: wfx: drop unnecessary wvif->powersave_mode

staging: wfx: drop unnecessary wvif->powersave_mode

Power save status is already available in bss_conf. So there is no
reason to keep information duplicated in wvif->powersave_mode.

In add, type of wvif->powersave_mode is low level struct made to
communicate with device. We would like to limit usage of this kind of
struct in upper layers of the driver.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-35-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>