staging: comedi: s626: fix async command hookup

Only set the SDF_CMD_READ subdev_flag if the irq was sucessfully
requested.

Remove the unnecessary sanity check of dev->irq in s626_ai_cmd().
This callback is only hooked up if the irq is available.

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