mac80211: fix sta_info_flush() return value

The comment for sta_info_flush() states
"Returns the number of removed STA entries"
but that isn't actually true. Consequently,
the warning when a station is still around
on interface removal can never trigger and
this delayed finding the timer issue the
previous patch fixed. Fix the return value
here to make that warning useful again.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>