Bluetooth: Add USB autosuspend support to btusb driver

This patch adds support of USB autosuspend to the btusb driver.

If the device doesn't support remote wakeup, simple support based on
up/down is provided. If the device supports remote wakeup, additional
support for autosuspend while the interface is up is provided. This is
done by queueing URBs in an anchor structure and waking the device up
from a work queue on sending. Reception triggers remote wakeup.

The last busy facility of the USB autosuspend code is used. To close
a race between autosuspend and transmission, a counter of ongoing
transmissions is maintained.

Add #ifdefs for CONFIG_PM as necessary.

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Tested-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>