wext: add back wireless/ dir in sysfs for cfg80211 interfaces

The move away from having drivers assign wireless handlers,
in favour of making cfg80211 assign them, broke the sysfs
registration (the wireless/ dir went missing) because the
handlers are now assigned only after registration, which is
too late.

Fix this by special-casing cfg80211-based devices, all
of which are required to have an ieee80211_ptr, in the
sysfs code, and also using get_wireless_stats() to have
the same values reported as in procfs.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reported-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Tested-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>