riscv: Fix 32-bit RISC-V boot failure

Commit dd2d082b ("riscv: Cleanup setup_bootmem()") adjusted
the calling sequence in setup_bootmem(), which invalidates the fix
commit de043da0 ("RISC-V: Fix usage of memblock_enforce_memory_limit")
did for 32-bit RISC-V unfortunately.

So now 32-bit RISC-V does not boot again when testing booting kernel
on QEMU 'virt' with '-m 2G', which was exactly what the original
commit de043da0 ("RISC-V: Fix usage of memblock_enforce_memory_limit")
tried to fix.

Fixes: dd2d082b

 ("riscv: Cleanup setup_bootmem()")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>