usb: device: fix Bluetooth buffer handling

In both implementation, when comparing received data length take into
account that the buffer obtained from bt_buf_get_tx() stores the type at
the top. The buffer types are H:4 and in the TX path we need to check for
BT_HCI_H4_* types not BT_BUF_*.

In the legacy implementation, the hci_acl_pkt_len() function obtains the
length from the USB transaction buffer, which does not contain a buffer
type at the top.

In the new implementation, partially revert the changes and restore
hci_pkt_get_len(), this will be required for any further changes anyway.

Fixes commit f85d63a6 ("Bluetooth: Remove USB H4 mode support").

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>