usb: appledisplay: Remove unnecessary spinlock

The spinlock was inside the urb completion function which is only
called once per display and is then resubmitted from this function.
There was no other place where this lock was used.

Signed-off-by: Alexander Theissen <alex.theissen@me.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>