dt/fsldma: fix build warning caused by of_platform_device changes

Commit 000061245a6797d542854106463b6b20fbdcb12e, "dt/powerpc:
Eliminate users of of_platform_{,un}register_driver" forgot to convert
the type of structure passed into platform_device_register() when it
was converted from of_platform_device_register. Fix it.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>