nl80211: remove nl80211_prepare_wdev_dump() skb argument

nl80211_prepare_wdev_dump() is using the output skb to look up
the network namespace, but this isn't really necessary, it can
just as well use the input skb which is available as cb->skb,
the sk is the same anyway.

Therefore, remove the redundant argument.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>