staging: comedi: vmk80xx: factor out usb buffer allocation

Factor the code that allocates the usb buffers out of vmk80xx_usb_probe().

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