Bluetooth: qca: Fix BT enable failure again for QCA6390 after warm reboot

Commit 272970be ("Bluetooth: hci_qca: Fix driver shutdown on closed
serdev") will cause below regression issue:

BT can't be enabled after below steps:
cold boot -> enable BT -> disable BT -> warm reboot -> BT enable failure
if property enable-gpios is not configured within DT|ACPI for QCA6390.

The commit is to fix a use-after-free issue within qca_serdev_shutdown()
by adding condition to avoid the serdev is flushed or wrote after closed
but also introduces this regression issue regarding above steps since the
VSC is not sent to reset controller during warm reboot.

Fixed by sending the VSC to reset controller within qca_serdev_shutdown()
once BT was ever enabled, and the use-after-free issue is also fixed by
this change since the serdev is still opened before it is flushed or wrote.

Verified by the reported machine Dell XPS 13 9310 laptop over below two
kernel commits:
commit e00fc2700a3f ("Bluetooth: btusb: Fix triggering coredump
implementation for QCA") of bluetooth-next tree.
commit b23d98d4 ("Bluetooth: btusb: Fix triggering coredump
implementation for QCA") of linus mainline tree.

Fixes: 272970be

 ("Bluetooth: hci_qca: Fix driver shutdown on closed serdev")
Cc: stable@vger.kernel.org
Reported-by: Wren Turkal <wt@penguintechs.org>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218726
Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Tested-by: Wren Turkal <wt@penguintechs.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>