usb: gadget: u_serial: fix typo which cause build warning

fix typo error introduced by commit ea0e6276 (usb: gadget: add
multiple definition guards) which causes the following build warning:

  warning: "pr_vdebug" redefined
Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/u_serial.c

@@ -136,7 +136,7 @@ static struct portmaster {
 	pr_debug(fmt, ##arg)
 #endif /* pr_vdebug */
 #else
-#ifndef pr_vdebig
+#ifndef pr_vdebug
 #define pr_vdebug(fmt, arg...) \
 	({ if (0) pr_debug(fmt, ##arg); })
 #endif /* pr_vdebug */