can: mcp251x: Use DT-supplied interrupt flags

The MCP2515 datasheet clearly describes a level-triggered interrupt pin.
Therefore the receiving interrupt controller must also be configured for
level-triggered operation otherwise there is a danger of a missed
interrupt condition blocking all subsequent interrupts. The ONESHOT
flag ensures that the interrupt is masked until the threaded interrupt
handler exits.

Rather than change the flags globally (they must have worked for at
least one user), keep the old behavior for for non DT devices. DT based
devices specify the flags in their corresonding DT node.

See: https://github.com/raspberrypi/linux/issues/2175
     https://github.com/raspberrypi/linux/issues/2263



Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Tested-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>