xen/riscv: Fix SSTC init to write VSTIMECMP instead of STIMECMP

According to the preceeding comment, CSR_VSTIMECMP should be written rather
than CSR_STIMECMP.

Fixes: 25e0327306 ("xen/riscv: allow Xen to use SSTC while hiding it from guests")
Signed-off-by: Baptiste Le Duc <baptiste.le-duc@vates.tech>
Reviewed-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/arch/riscv/time.c

@@ -101,6 +101,6 @@ void __init preinit_xen_time(void)
          * Thereby to avoid spurious VS-timer irqs set vstimecmp CSR to
          * ULONG_MAX.
          */
-        csr_write64(CSR_STIMECMP, ULONG_MAX);
+        csr_write64(CSR_VSTIMECMP, ULONG_MAX);
     }
 }