lib: sbi: fix ctz bug

The original version of ctz will cause an endless loop, if the parameter
passed in is 0. This commit fixes this bug.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

lib/sbi/riscv_asm.c

@@ -213,6 +213,9 @@ static unsigned long ctz(unsigned long x)
 {
 	unsigned long ret = 0;
 
+	if (x == 0)
+		return 8 * sizeof(x);
+
 	while (!(x & 1UL)) {
 		ret++;
 		x = x >> 1;