mcp2515: 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), allow the flags to be overridden from Device Tree
in the event that the device has a 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>