rtw88: rtw8821c: enable rfe 6 devices

Ping-Ke Shih answered[1] a question for a user about an rtl8821ce device that
reported RFE 6, which the driver did not support. Ping-Ke suggested a possible
fix, but the user never reported back.

A second user discovered the above thread and tested the proposed fix.
Accordingly, I am pushing this change, even though I am not the author.

[1] https://lore.kernel.org/linux-wireless/3f5e2f6eac344316b5dd518ebfea2f95@realtek.com/

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Reported-and-tested-by: masterzorag <masterzorag@gmail.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220107024739.20967-1-Larry.Finger@lwfinger.net

drivers/net/wireless/realtek/rtw88/rtw8821c.c

@@ -1514,6 +1514,7 @@ static const struct rtw_rfe_def rtw8821c_rfe_defs[] = {
 	[0] = RTW_DEF_RFE(8821c, 0, 0),
 	[2] = RTW_DEF_RFE_EXT(8821c, 0, 0, 2),
 	[4] = RTW_DEF_RFE_EXT(8821c, 0, 0, 2),
+	[6] = RTW_DEF_RFE(8821c, 0, 0),
 };
 
 static struct rtw_hw_reg rtw8821c_dig[] = {