mac80211: fix ethtool stats for non-station interfaces

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.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>