usb: serial: fix memory leak in generic driver

Fix a regression introduced by commit
715b1dc0 ("USB: usb_debug,
usb_generic_serial: implement multi urb write").

URB transfer buffer was never freed when using multi-urb writes. 
Currently the only driver enabling multi-urb writes is usb_debug.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: Greg KH <greg@kroah.com>
Acked-by: Jason Wessel <jason.wessel@windriver.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>