nl80211: move "can set channel" check

Setting the wdev to NULL when the channel can't be
set for that interface type (to treat the channel
setting for the wiphy/monitor) currently works, but
is confusing in the code if netdev/wdev aren't both
set/unset in the same way. Move the check whether
the channel can be set to where it's needed so that
wdev and netdev are always both assigned or NULL.

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