staging: comedi: ni_6527: tidy up the interrupt subdevice

Do the request_irq() before setting up the subdevices. Only initialize
the interrupt subdevice if the irq is actually available.

Tidy up the whitespace in the subdevice init.

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>