usb: musb: cppi41: allow it to work again

since commit 33c300cb ("usb: musb: dsps:
don't fake of_node to musb core") we have been
preventing CPPI 4.1 from probing due to NULL
of_node. We can't revert said commit otherwise
a different regression would show up, so the fix
is to look for the parent device's (glue layer's)
of_node instead, since that's the thing which
is actually described in DTS.

Signed-off-by: Felipe Balbi <balbi@ti.com>