WHENCE: link to similar config file for rtl8821a support

linux-firmware ships no config file for rtl8821a, as it seems
quite similar to rtl8821c add symlink to that config file
see https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1784835
(Maintainers may replace with proper file).

fixes: https://bugs.debian.org/893912

-- error
[    2.574742] Bluetooth: hci0: rtl: loading rtl_bt/rtl8821a_config.bin
[    2.575137] bluetooth hci0: firmware: failed to load
rtl_bt/rtl8821a_config.bin (-2)
[    2.575143] bluetooth hci0: Direct firmware load for
rtl_bt/rtl8821a_config.bin failed with error -2
[    2.575145] Bluetooth: hci0: Failed to load rtl_bt/rtl8821a_config.bin
--

-- with config file
[   26.389532] Bluetooth: hci0: RTL: rtl: examining hci_ver=06
hci_rev=000a lmp_ver=06 lmp_subver=8821

[   26.390623] Bluetooth: hci0: RTL: rom_version status=0 version=1

[   26.390624] Bluetooth: hci0: RTL: rtl: loading rtl_bt/rtl8821a_fw.bin

[   26.392597] bluetooth hci0: firmware: direct-loading firmware
rtl_bt/rtl8821a_fw.bin
[   26.392605] Bluetooth: hci0: RTL: rtl: loading rtl_bt/rtl8821a_config.bin

[   26.392748] bluetooth hci0: firmware: direct-loading firmware
rtl_bt/rtl8821a_config.bin
[   26.392757] Bluetooth: hci0: RTL: cfg_sz 10, total sz 17438
--

Cc: Max Chou <max.chou@realtek.com>
Reported-by: Kate Komi <katahipo@gmail.com>
Signed-off-by: maximilian attems <maks@stro.at>

WHENCE

@@ -3731,6 +3731,7 @@ Link: rtl_bt/rtl8723bs_config-OBDA0623.bin -> rtl8723bs_config-OBDA8723.bin
 File: rtl_bt/rtl8761a_fw.bin
 File: rtl_bt/rtl8812ae_fw.bin
 File: rtl_bt/rtl8821a_fw.bin
+Link: rtl_bt/rtl8821a_config.bin -> rtl8821c_config.bin
 File: rtl_bt/rtl8822b_fw.bin
 File: rtl_bt/rtl8822b_config.bin
 File: rtl_bt/rtl8723d_fw.bin