powerpc/kernel: Avoid initializing device-tree pointer twice

As commit 50ba08f3

 ("of/fdt: Don't clear initial_boot_params
if fdt_check_header() fails") does, the device-tree pointer
"initial_boot_params" is initialized by early_init_dt_verify(),
which is called by early_init_devtree(). So we needn't explicitly
initialize that again in early_init_devtree().

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>