staging: comedi: ni_6527: 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.  The `INSN_CONFIG_DIGITAL_TRIG` comedi instruction
is more flexible as it supports both edge and level detection, but is
not currently supported by this driver.

Add partial support for `INSN_CONFIG_DIGITAL_TRIG`, but only for edge
detection.  Make use of the `ni6527_set_edge_detection()` used for
`INSN_CONFIG_CHANGE_NOTIFY`, but add a parameter holding a mask of the
rising and falling edges to be updated and preserve the unmasked edges
when updating.

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