riscv: Do not overwrite initrd_start and initrd_end

setup_initrd() overwrites initrd_start and initrd_end if __initramfs_size
is larger than 0, which is always true even if there is no embedded
initramfs. This prevents booting qemu with "-initrd" parameter.
Overwriting initrd_start and initrd_end is not necessary since
__initramfs_start and __initramfs_size are used directly in
populate_rootfs() to load the built-in initramfs, so just drop
that code.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>