disable the regular ieee80211_encap() Tx path in monitor mode

disable the regular ieee80211_encap() Tx path in monitor mode

Frames injected from user space carry the DLT_IEEE802_11_RADIO
mbuf tag, and are handled as a special case. Do not fall back
to regular encapsulation while we are in monitor mode and the
frame injected by userspace is found to be invalid.

This fixes an issue when iwx(4) runs in monitor mode with addresses
configured on the interface and leaving 11n/11ac mode directly for
monitor mode. In this case, traffic generated by userspace or the
kernel (such as ICMPv6) would trigger Tx attempts, which in turn
would trigger an attempt to set up a block ACK agreement and then
cause a firmware panic.

This points at a related issue where interface configuration state
is not properly cleaned up while switching into monitor mode.
The 11n/11ac interface config should ideally be cleared completely,
preventing block ack from being initiated.

But preventing the stack from trying to send frames down the regular
Tx path in monitor mode is a good idea in general because drivers may
not handle this very well for various reasons, block ack being just one.

tested by jmc@ and myself on iwx ax200