usb: dwc3: gadget: update DCFG.NumP to max burst size

NumP field of DCFG register is used on NumP field of
ACK TP header and it tells the host how many packets
an endpoint can receive before waiting for
synchronization.

Documentation says it should be set to anything
<=bMaxBurst. Interestingly, however, this setting is
not per-endpoint how it should be (different
endpoints could have different burst sizes), but
things seem to work okay right now.

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