drivers: bluetooth: hci: Increase default RX stack size

The `BT_DRV_RX_STACK_SIZE` config is used by HCI drivers to determine a
suitable stack size for their RX thread. These threads handle the HCI
transport and pass HCI messages to the Bluetooth host via `bt_hci_recv()`.
Previously, the default stack size was 256 bytes, but it seems to be too
small currently.

Measuring the peak stack usage of `bt_hci_recv()` in different scenarios
indicates that the function needs over 300 bytes of stack. Thus, an RX
thread stack size of 512 should cover that and leave some margin.

Signed-off-by: Kalle Kietäväinen <kalle.kietavainen@silabs.com>