usb: dwc3: switch trb enqueue/dequeue and first_trb_index to u8

We *know* that we have 1 PAGE (4096 bytes) for our
TRB poll. We also know the size of each TRB and know
that we can fit 256 of them in one PAGE. By using a
u8 type we can make sure that:

	enqueue++ % 256;

gets optimized to an increment only.

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