riscv: signal: fix some warnings reported by sparse

riscv: signal: fix some warnings reported by sparse

Clean up a few warnings reported by sparse in
arch/riscv/kernel/signal.c.  These come from code that was added
recently; they were missed when I initially reviewed the patch.

Fixes: 818d78ba1b3f ("riscv: signal: abstract header saving for setup_sigcontext")
Cc: Andy Chiu <andybnac@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202601171848.ydLTJYrz-lkp@intel.com/
[pjw@kernel.org: updated to apply]
Signed-off-by: Paul Walmsley <pjw@kernel.org>