usb: dwc3: gadget: rename busy/free_slot to trb_enqueue/dequeue

This makes it clear that we're dealing with a queue
of TRBs. No functional changes. While at that, also
rename start_slot to first_trb_index for similar
reasons.

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