usb: gadget: rndis: fix itnull.cocci warnings

The index variable of list_for_each_entry_safe is an offset from a list
pointer, and thus should not be NULL.

Generated by: scripts/coccinelle/iterators/itnull.cocci

CC: Geliang Tang <geliangtang@163.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>