staging: comedi: add comedi_usb.h

Add a new header that Comedi USB drivers can include instead of
"comedidev.h".  Currently, it just pulls in <linux/usb.h> and
"comedidev.h", but the plan is to migrate the USB-specific stuff from
"comedidev.h" here.

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>