mrf24j40: Use level-triggered interrupts

The mrf24j40 generates level interrupts. There are rare cases where it
appears that the interrupt line never gets de-asserted between interrupts,
causing interrupts to be lost, and causing a hung device from the driver's
perspective.  Switching the driver to interpret these interrupts as
level-triggered fixes this issue.

Signed-off-by: Alan Ott <alan@signal11.us>
Signed-off-by: David S. Miller <davem@davemloft.net>