mac80211: fix ethtool stats for non-station interfaces

commit e13bae4f807401729b3f27c7e882a96b8b292809 upstream.

As reported in https://bugzilla.kernel.org/show_bug.cgi?id=60514

,
the station loop never initialises 'sinfo' and therefore adds up
a stack values, leaking stack information (the number of times it
adds values is easily obtained another way.)

Fix this by initialising the sinfo for each station to add.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>