msc examples: Extend serial number to 12 digits

USB3CV tool complains about too short serial number being used
in MSC device.

This just extends serial number to 12 characters, it makes it
easier to analyze USB3CV logs where this unnecessary warning
made output red.

Offending warning:
Serial Number string for MSC device : iSerialNumber = 0x3
Checking iSerialNumber String Descriptor: index = 0x03.
String Descriptor : "123456". (ENGLISH_US)
Using Language ID 0x409
MSC Serial Number length = 14
Invalid MSC Serial Number length : should be >= 26
*************************
Invalid MSC Serial Number length
*************************
*************************
(MSC: 5.1.2) Serial number must be a string, 12 characters or longer
(if the device supports a BOT interface, bInterfaceProtocol = 0x50),
or exactly 12 characters long (if the device supports a CBI interface,
bInterfaceProtocol = 0x00 or 0x01, and has a serial number).

examples/device/msc_dual_lun/src/usb_descriptors.c

@@ -141,7 +141,7 @@ char const* string_desc_arr [] =
   (const char[]) { 0x09, 0x04 }, // 0: is supported language is English (0x0409)
   "TinyUSB",                     // 1: Manufacturer
   "TinyUSB Device",              // 2: Product
-  "123456",                      // 3: Serials, should use chip ID
+  "123456789012",                // 3: Serials, should use chip ID
 };
 
 static uint16_t _desc_str[32];