Bluetooth: btusb: Separate TX URB allocation and submission

The complete TX URB handling is done via a switch statement in the
btusb_send_frame function. To allow for more clear separation between
control, bulk and isoc URBs, split them into allocation and submission.

Previously the inc_tx function has been used for tracking in-flight
URB for HCI commands and ACL data packets. Convert that into a common
function that either submits the URB or queues it when needed.

This provides the flexibility to allow vendor specific hdev->send_frame
callbacks without having to duplicate the whole URB handling logic.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>