usb: dwc3: gadget: dynamically re-size TxFifos

We need to dynamically re-size TxFifos for the
cases where default values will not do.

While at that, we create a simple function which,
for now, will just allocate one full packet fifo
space for each of the enabled endpoints.

This can be improved later in order to allow for
better throughput by allocating more space for
endpoints which could make good use of that like
isochronous and bulk.

Signed-off-by: Felipe Balbi <balbi@ti.com>