usb: gadget: fix request length error for isoc transfer

For isoc endpoint descriptor, the wMaxPacketSize is not real max packet
size (see Table 9-13. Standard Endpoint Descriptor, USB 2.0 specifcation),
it may contain the number of packet, so the real max packet should be
ep->desc->wMaxPacketSize && 0x7ff.

Cc: Felipe F. Tonello <eu@felipetonello.com>
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Fixes: 16b114a6 ("usb: gadget: fix usb_ep_align_maybe
  endianness and new usb_ep_aligna")

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>