usb: dwc3: gadget: Properly initialize LINK TRB

On ISOC endpoints the last trb_pool entry used as a
LINK TRB is not getting zeroed out correctly due to
memset being called incorrectly and in the wrong place.
If pool allocated from DMA was not zero-initialized
to begin with this will result in the size and ctrl
values being random garbage. Call memset correctly after
assignment of the trb_link pointer.

Change-Id: I6fece42f1cde6e66297e01307b7b6a362adacc79
Signed-off-by: Jack Pham <jackp@codeaurora.org>