staging/comedi/drivers/usbduxsigma.c: added support for ehci drivers

staging/comedi/drivers/usbduxsigma.c: added support for ehci drivers

urb->interval is deprecated and thus I've changed the driver
that it now always assumes interval=1 which means every frame
in USB 1.1 and every uframe in USB 2.0. However we still need
to have different sampling rates which are still multiples
of the interval which is now transmitted to the firmware.
The firmware transmits either zero length packets or none every (u)frame.
This is checked in the completion handler and any packet
at zero length is discarded so that comedi again sees the data
coming in at the interval specified. This also then gives the ADC
the necessary time to convert. For example 16 channels require
about 700us and in this period no packet could be transmitted.
In this case this is padded up to 1ms so that we have then 7 zero
length packets and one packet with the ADC data.

Signed-off-by: Bernd Porr <mail@berndporr.me.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>