wifi: rtw89: debug: add mlo_mode dbgfs

Add an dbgfs mlo_mode to get/set MLO mode. And, support to trigger MLSR
switching. Setting it will automatically disable MLO Dynamic Mechanism
(DM). Then MLO things can follow commands via dbgfs mlo_mode instead of
MLO DM. The disabled DM(s) can be checked/cleared via dbgfs disable_dm.

The following is an use example.

Read it to show current MLD status.
	$ cat mlo_mode

	MLD(s) status: (MLO DM: enable)
		#0: MLO mode 0, valid 0x6, active 0x2

Write it to switch to MLSR on link id 2.
	$ echo 0 2 > mlo_mode
	$ cat mlo_mode

	MLD(s) status: (MLO DM: disable)
		#0: MLO mode 0, valid 0x6, active 0x4

Besides, to be safer, add RWLOCK attribute to dbgfs disable_dm.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250505072440.45113-11-pkshih@realtek.com