staging: comedi: usbdux: use 'cmd' pointer in usbduxsub_ai_isoc_irq()

Use the 'cmd' pointer to access the chanlist instead of getting to it
using the comedi_subdevice pointer.

Remove the unneeded 'n' local variable, this is just the cmd->chanlist_len.

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>