staging: comedi: ni_65xx: support INSN_CONFIG_DIGITAL_TRIG

The "edge detection interrupt" subdevice supports the
`INSN_CONFIG_CHANGE_NOTIFY` comedi instruction which is only supported
by one other driver.  It is limited to the first 32 channels, but boards
supported by this driver support edge detection on all digital I/O,
digital input and digital output channels.  The
`INSN_CONFIG_DIGITAL_TRIG` comedi instruction is more flexible as it
supports more than 32 channels (with multiple instructions).  It can
also support level detection, but the hardware does not support that.

Add partial support for the `INSN_CONFIG_DIGITAL_TRIG` instruction, but
only for edge detection.

Since `INSN_CONFIG_CHANGE_NOTIFY` can only deal with 32 channels, make
it disable edge detection for any remaining channels.

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