mac80211: check per vif offload_flags in Tx path

offload_flags has been introduced to indicate encap status of each interface.
An interface can encap offload at runtime, or if it has some extra limitations
it can simply override the flags, so it's more flexible to check offload_flags
in Tx path.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://lore.kernel.org/r/177785418cf407808bf3a44760302d0647076990.1623961575.git.ryder.lee@mediatek.com


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