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

Fixes: 83374e03 ("usb: mtu3: add tracepoints to help debug")
Cc: stable@vger.kernel.org
Reported-by: NYuwen Ng <yuwen.ng@mediatek.com>
Signed-off-by: NChunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20211218095749.6250-3-chunfeng.yun@mediatek.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/mtu3/mtu3_gadget.c

@@ -235,6 +235,7 @@ struct usb_request *mtu3_alloc_request(struct usb_ep *ep, gfp_t gfp_flags)
 	mreq->request.dma = DMA_ADDR_INVALID;
 	mreq->epnum = mep->epnum;
 	mreq->mep = mep;
+	INIT_LIST_HEAD(&mreq->list);
 	trace_mtu3_alloc_request(mreq);
 
 	return &mreq->request;