Revert "usb: dwc3: gadget: always decrement by 1"

This reverts commit 6f8245b4 ("usb: dwc3: gadget: always decrement
by 1").

We can't always decrement this value.

We should decrement only if the calculation of free slots results in a
LINK TRB being among one of the free slots (dequeue < enqueue).

Otherwise, if the LINK TRB is not among the free slots then it should
not be decremented.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>