spi/mpc83xx: fix NULL pdata dereference bug

Commit 178db7d3, "spi: Fix device unregistration when unregistering
the bus master", changed device initialization to be children of the
bus master, not children of the bus masters parent device. The pdata
pointer used in fsl_spi_chipselect must updated to reflect the changed
initialization.

Signed-off-by: Kenth Eriksson <kenth.eriksson@transmode.com>
Acked-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>