usb/core: remove "always" from usb_unlink_urb() kernel doc entry

The kernel doc entry for usb_unlink_urb() contains the phrase "This
request is always asynchronous.". The "always" leads to the assumption
that the ->complete() callback is not called from within
usb_unlink_urb(). This is not true. The HCD is allowed to call the
->complete() from within ->urb_dequeue() if it is appropriate for the
hardware.
This patch updates the kernel doc so usb-device driver authors make sure
to drop all locks (and make sure it is okay to drop them) which are
acquired by the complete callback before calling usb_unlink_urb().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>