usb/early: remove set but not used variable 'remain_length'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/usb/early/xhci-dbc.c: In function 'xdbc_handle_tx_event':
drivers/usb/early/xhci-dbc.c:720:9: warning:
 variable 'remain_length' set but not used [-Wunused-but-set-variable]

It never be used since introduction in
commit aeb9dd1d ("usb/early: Add driver for xhci debug capability")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/early/xhci-dbc.c

@@ -717,17 +717,14 @@ static void xdbc_handle_port_status(struct xdbc_trb *evt_trb)
 
 static void xdbc_handle_tx_event(struct xdbc_trb *evt_trb)
 {
-	size_t remain_length;
 	u32 comp_code;
 	int ep_id;
 
 	comp_code	= GET_COMP_CODE(le32_to_cpu(evt_trb->field[2]));
-	remain_length	= EVENT_TRB_LEN(le32_to_cpu(evt_trb->field[2]));
 	ep_id		= TRB_TO_EP_ID(le32_to_cpu(evt_trb->field[3]));
 
 	switch (comp_code) {
 	case COMP_SUCCESS:
-		remain_length = 0;
 	case COMP_SHORT_PACKET:
 		break;
 	case COMP_TRB_ERROR: