staging: comedi: usbduxfast: use a common exit path in ai (*do_cmd)

staging: comedi: usbduxfast: use a common exit path in ai (*do_cmd)

For aesthetics, use 'goto' and a common exit point in this function so
that the up of the semaphore is not overlooked.

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