riscv: fix building with CONFIG_SPL_SMP=n

Building with CONFIG_SPL_SMP=n results in:

arch/riscv/lib/spl.c: In function ‘jump_to_image_no_args’:
arch/riscv/lib/spl.c:33:6:
error: unused variable ‘ret’ [-Werror=unused-variable]
   33 |  int ret;
      |      ^~~

Define the variable ret as __maybe_unused.

Fixes: 191636e4 ("riscv: Introduce SPL_SMP Kconfig option for U-Boot SPL")
Fixes: 8c59f202 ("riscv: add SPL support")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@openfive.com>