rockchip: puma_rk3399: increase serialno_str size

Increase serialno_str to 17 bytes so it can hold the 16 bytes long serial
nummer and the terminating null byte added by snprintf.

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
diff --git a/board/theobroma-systems/puma_rk3399/puma-rk3399.c b/board/theobroma-systems/puma_rk3399/puma-rk3399.c
index c6f8eed..45d56cd 100644
--- a/board/theobroma-systems/puma_rk3399/puma-rk3399.c
+++ b/board/theobroma-systems/puma_rk3399/puma-rk3399.c
@@ -107,7 +107,7 @@
 	u8 low[cpuid_length/2], high[cpuid_length/2];
 	char cpuid_str[cpuid_length * 2 + 1];
 	u64 serialno;
-	char serialno_str[16];
+	char serialno_str[17];
 
 	/* retrieve the device */
 	ret = uclass_get_device_by_driver(UCLASS_MISC,