Bluetooth: btrtl: Ask 8821C to drop old firmware

Some platforms keep USB power even when they are powered off and in S5,
this makes Realtek 8821C keep its firmware even after a cold boot, and
make 8821C never load new firmware.

So use vendor specific HCI command to ask 8821C drop its firmware after
system shutdown.

Newer firmware doesn't have this issue so we only use this trick for old
8821C firmware version.

Suggested-by: Max Chou <max.chou@realtek.com>
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>