staging: comedi: addi_apci_2032: interrupt safety change

Put the hardware into a safe state before enabling the interrupt.  In
the interrupt routine, check the device has been fully configured by
checking `dev->attached`.  In particular, `dev->read_subdev` could be
NULL early on and although the hardware's status register should
indicate no interrupt has occurred (since it's been put into a safe
state), it's better not to rely on it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>