usb: gadget: udc: Use scnprintf() instead of snprintf()

The show() method should use scnprintf() not snprintf() because snprintf()
may returns a value that exceeds its second argument.

Signed-off-by: Jaejoong Kim <climbbb.kim@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>