Bluetooth: Support `BT_RECV_WORKQ_BT` with Zephyr LL

This patch allows selecting the combination of `CONFIG_BT_RECV_WORKQ_BT`
and `BT_LL_SW_SPLIT`. The implementation of `bt_recv_prio` is copied
from `hci_raw.c`. This ensures the order of packets is the same as when
the controller's `hci_driver.c` is off-chip and sends HCI over UART.

Without this patch, the Zephyr LL driver will duplicate some events on
behalf of `hci_core`. This duplication is not wanted when
`CONFIG_BT_RECV_WORKQ_BT=y`.

The above mentioned duplication is not sound and is evidenced in logged
warnings, e.g. in `tests/bsim/bluetooth/host/l2cap/stress` with
`CONFIG_BT_RECV_WORKQ_BT=y` logging "no transition".

This patch is a step towards the deprecation of
`CONFIG_BT_RECV_BLOCKING`. `CONFIG_BT_RECV_BLOCKING` is complicated and
tightly coupled to `hci_core`. In the future, removing
`CONFIG_BT_RECV_BLOCKING` will give a cleaner separation between
`hci_core` and the drivers and allow `hci_core` to evolve without
changes spilling out into the drivers.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>