gadget: Use vsprintf extention %pf with builtin_return_address

Emit the function name not the address when possible.

builtin_return_address() gives an address.  When building
a kernel with CONFIG_KALLSYMS, emit the actual function
name not the address.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/usb/gadget/u_serial.c

@@ -920,7 +920,7 @@ static int gs_put_char(struct tty_struct *tty, unsigned char ch)
 	unsigned long	flags;
 	int		status;
 
-	pr_vdebug("gs_put_char: (%d,%p) char=0x%x, called from %p\n",
+	pr_vdebug("gs_put_char: (%d,%p) char=0x%x, called from %pf\n",
 		port->port_num, tty, ch, __builtin_return_address(0));
 
 	spin_lock_irqsave(&port->port_lock, flags);