staging: comedi: pcl711: don't fail attach if irq is unavailable

Interrupts are only needed to support asynchronous commands with the
analog input subdevice. Since the interrupt is optional during the
attach of the board, don't fail the attach if it's not available.

Also, remove the printk() noise about the irq. The user will know if
the interrupt is available due to the subdev_flag SDF_CMD_READ being
set for the analog input subdevice.

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>