powerpc: fsl_msi doesn't need it's own of_node

The FSL MSI code keeps a pointer to the of_node from the device
it represents.  However it also has an irq_host, which contains
a pointer to the of_node, so use that one instead.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>