wifi: rtw89: regd: introduce string getter for reuse

Introduce a function to get the string for a given regulatory. It will be
used in the following. Besides, drop similar things in debug code and use
this too.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250326020643.14487-3-pkshih@realtek.com