mac80211: Allow concurrent monitor iface and ethernet rx decap

Some HW/driver can support passing ethernet rx decap frames and
raw 802.11 frames for the monitor interface concurrently and
via separate RX calls to mac80211. Packets going to the monitor
interface(s) would be in 802.11 format and thus not have the
RX_FLAG_8023 set, and 802.11 format monitoring frames should have
RX_FLAG_ONLY_MONITOR set.

Drivers doing such can enable the SUPPORTS_CONC_MON_RX_DECAP to
allow using ethernet decap offload while a monitor interface is
active, currently RX decapsulation offload gets disabled when a
monitor interface is added.

Signed-off-by: Sriram R <srirrama@codeaurora.org>
Link: https://lore.kernel.org/r/1617068116-32253-1-git-send-email-srirrama@codeaurora.org


[add proper documentation, rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>