usb: dwc3: gadget: lazily map requests for DMA

Some functions might want to have very, very long request queues. We
can't make any assumptions about how many requests we *are* able to
map, so instead of mapping requests early, let's map them late. This
way, functions can queue as many requests as they'd like but we won't
take DMA resources until they are needed.

Also, we can now stop processing requests when we run out of DMA
resources but still keep requests in the queue for late processing.

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