staging: wfx: drop counter of buffered frames

Since the driver does not call ieee80211_sta_set_buffered() anymore, it
is no more necessary to maintain a counter of buffered frames for each
stations.

This change allows to simplify the processing in multiple places in the
driver.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200701150707.222985-9-Jerome.Pouiller@silabs.com


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