usb: dwc3: gadget: fix skip LINK_TRB on ISOC

When we reach to link trb, we just need to increase free_slot and then
calculate TRB. Return is not correct, as it will cause wrong TRB DMA
address to fetch in case of update transfer.

Cc: <stable@vger.kernel.org>
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>