wifi: mac80211_hwsim: fix multi-channel handling in netlink RX

In netlink RX, now that we can actually have multiple
channel contexts for MLO, things don't work well as we
only keep a single pointer, and then on link switching
we might NULL it, and hit the return if the channel is
NULL.

However, we already use mac80211_hwsim_tx_iter() which
deals with all this, so remove the test and adjust the
remaining code a bit.

This then means we no longer use the chanctx pointer,
so remove it as well.

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