staging: comedi: split out USB support into new module

staging: comedi: split out USB support into new module

Setting the `CONFIG_COMEDI_USB_DRIVERS` kernel configuration option
makes the main "comedi" module depend on the "usbcore" module.  But
perhaps the machine has no USB controllers (or they have been disabled),
in which case the "usbcore" module may have been pulled in
unnecessarily.  Only a few low-level Comedi drivers require USB support.
The Comedi USB support functions are all in "comedi_usb.c".  Turn it
into a separate module so we don't have to pull in the "usbcore" and
"usb_common" modules unnecessarily.

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>