staging: comedi: usbduxfast: adding missing break in case statement

Added a missing "break" which forced the board to acquire 16
channels even when only 3 had been requested.
Thanks for Hartley Sweeten to spot this bug.

Signed-off-by: Bernd Porr <mail@berndporr.me.uk>
Reported-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>