Bluetooth: Host: Fix bt_disable() for IPC giving HCI Reset timeout

Some HCI drivers issue HCI reset when disabling, like the IPC HCI
driver. We need to keep the RX thread running to allow receiving
the command complete.

This commit postpones aborting the RX thread until this is done.
The issue happens started occuring after commit
d0e75ab87c4b53d66008c941c38709a2fca9dbea.

Fixes #76202.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>