wifi: rtw89: chan: re-config default chandef only when none is registered

Previously, default chandef is configured if no chanctx is active, i.e. no
chanctx is assigned to some vif. For normal cases, it's fine. However, for
impending CSA support, need to consider that one chanctx may be added, or
called registered, ahead without being assigned immediately. Then, it will
keep inactive, and might be covered by the default one when re-calculating
chanctxs happens in certain sequences. So now, don't re-config the default
chandef unless no chanctx is registered.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250605114207.12381-3-pkshih@realtek.com