wifi: mac80211: fix monitor channel with chanctx emulation

After the channel context emulation, there were reports that
changing the monitor channel no longer works. This is because
those drivers don't have WANT_MONITOR_VIF, so the setting the
channel always exits out quickly.

Fix this by always allocating the virtual monitor sdata, and
simply not telling the driver about it unless it wanted to.
This way, we have an interface/sdata to bind the chanctx to,
and the emulation can work correctly.

Cc: stable@vger.kernel.org
Fixes: 0a44dfc0

 ("wifi: mac80211: simplify non-chanctx drivers")
Reported-and-tested-by: Savyasaachi Vanga <savyasaachiv@gmail.com>
Closes: https://lore.kernel.org/r/chwoymvpzwtbmzryrlitpwmta5j6mtndocxsyqvdyikqu63lon@gfds653hkknl
Link: https://msgid.link/20240612122351.b12d4a109dde.I1831a44417faaab92bea1071209abbe4efbe3fba@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>