riscv/nsbi: unblock up_udelay

This fixes the always zero time in NuttSBI case as the `error` field
contains the correct timestamp. This in turn also fixes the block of
`up_udelay()` which further leads to boot failure of `canmv230:master`
config.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

arch/risc-v/src/common/supervisor/riscv_sbi.c

@@ -111,7 +111,7 @@ uint64_t riscv_sbi_get_time(void)
   sbiret_t ret = sbi_ecall(SBI_EXT_FIRMWARE, SBI_EXT_FIRMWARE_GET_MTIME,
                            0, 0, 0, 0, 0, 0);
 
-  return ret.value;
+  return ret.error;
 #elif defined(CONFIG_ARCH_RV64)
   return READ_CSR(CSR_TIME);
 #else