usbdux: usbduxsigma: added soft interval mode in the firmware

usbdux: usbduxsigma: added soft interval mode in the firmware

urb->interval is ignored in xhci drivers. Instead, the firmware
has an interval counter and transmits data at this interval.
The uframes in-between are zero length packets. In order to be
backward compatible with older kernels which support only USB 2.0
and 1.1 there are two ADC modes in the firmware. The old one which
assumes that the interval is established by the host (urb->interval)
and the new one where the interval value is transmitted to the
firmware.

Signed-off-by: Bernd Porr <mail@berndporr.me.uk>
Signed-off-by: Kyle McMartin <kyle@kernel.org>