nl80211: Use monitor interface for sending no-encrypt test frames

nl80211: Use monitor interface for sending no-encrypt test frames

Since NL80211_CMD_FRAME does not allow encryption to be disabled for the
frame, add a monitor interface temporarily for cases where this type of
no-encrypt frames are to be sent. The temporary monitor interface is
removed immediately after sending the frame.

This is testing functionality (only in CONFIG_TESTING_OPTIONS=y builds)
that is used for PMF testing where the AP can use this to inject an
unprotected Robust Management frame (mainly, Deauthentication or
Disassociation frame) even in cases where PMF has been negotiated for
the association.

Signed-off-by: Jouni Malinen <j@w1.fi>