mac80211: reorganize code to remove a forward declaration

Remove the newly added ieee80211_set_vif_encap_ops declaration.
No further code changes.
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20200908123702.88454-15-nbd@nbd.nameSigned-off-by: Johannes Berg <johannes.berg@intel.com>