wifi: rtw89: chan: allow callers to check if a link has no managed chanctx

wifi: rtw89: chan: allow callers to check if a link has no managed chanctx

Originally, to directly align with the chanctx design, getter of managed
chanctx returned a default channel when a link doesn't own a chanctx yet.
Then, callers could simply use the return without trivial NULL checking.

But in MLD HW settings of next chip, there will be a special case that a
caller needs to check if a link has owned chanctx or not to determine
CCK hardware circuit working on HW-x. So, add a func *_or_null for this
first.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250915065213.38659-2-pkshih@realtek.com