staging: comedi: ni_6527: tidy up ni6527_interrupt()

Rename the CamelCase defines used for the interrupt status register.

The NI6527_STATUS_IRQ bit will be set whenever the device is asserting
an interrupt. Modify the function a bit so this is the only requirement
for the interrupt to be IRQ_HANDLED. Currently an OVERFLOW interrupt is
not handled, though it's unlikely this would occur without an EDGE
interrupt.

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>