mac80211: fix some encapsulation offload kernel-doc

Add a missing kernel-doc entry for the offload_flags, and
correct the name of the update_vif_offload method.

Link: https://lore.kernel.org/r/20200918132115.d46a0915ba8a.Ibba536d04a5a5fb655f8ef6e51b247457bfda4ca@changeid


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