of/fdt: fix build warnings in early_init_dt_scan_chosen()

For some reason commit "of: fix CONFIG_CMDLINE_EXTEND" changed some
variable types, restore these to the old types to avoid the following
warnings:

drivers/of/fdt.c: In function 'early_init_dt_scan_chosen':
drivers/of/fdt.c:932:45: warning: passing argument 3 of 'of_get_flat_dt_prop' from incompatible pointer type
   p = of_get_flat_dt_prop(node, "bootargs", &l);
                                             ^
drivers/of/fdt.c:619:20: note: expected 'int *' but argument is of type 'long unsigned int *'
 const void *__init of_get_flat_dt_prop(unsigned long node, const char *name,
                    ^
drivers/of/fdt.c:932:5: warning: assignment discards 'const' qualifier from pointer target type
   p = of_get_flat_dt_prop(node, "bootargs", &l);
     ^

Signed-off-by: Jon Medhurst <tixy@linaro.org>