usb: mtu3: fix list_head check warning

In the Linux kernel, the following vulnerability has been resolved:

usb: mtu3: fix list_head check warning

This is caused by uninitialization of list_head.

BUG: KASAN: use-after-free in __list_del_entry_valid+0x34/0xe4

Call trace:

dump_backtrace+0x0/0x298

show_stack+0x24/0x34

dump_stack+0x130/0x1a8

print_address_description+0x88/0x56c

__kasan_report+0x1b8/0x2a0

kasan_report+0x14/0x20

__asan_load8+0x9c/0xa0

__list_del_entry_valid+0x34/0xe4

mtu3_req_complete+0x4c/0x300 [mtu3]

mtu3_gadget_stop+0x168/0x448 [mtu3]

usb_gadget_unregister_driver+0x204/0x3a0

unregister_gadget_item+0x44/0xa4