wext: allow returning NULL stats

Currently, wext drivers cannot return NULL for stats even though
that would make the ioctl return -EOPNOTSUPP because that would
mean they are no longer listed in /proc/net/wireless. This patch
changes the wext core's behaviour to list them if they have any
wireless_handlers, but only show their stats when available, so
that drivers can start returning NULL if stats are currently not
available, reducing confusion for e.g. IBSS.

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