staging: comedi: comedi_usb.h: move USB stuff out of comedidev.h

Move the USB-specific stuff out of "comedidev.h" into "comedi_usb.h".
Comedi USB drivers now include "comedi_usb.h" instead of "comedidev.h",
which now gets pulled in indirectly.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>