usb: gadget: ffs: Implement user URBs for USB bulk endpoints

URB aka USB Request Block: This patch offers similar functionality as
USBDEVFS_SUBMITURB / USBDEVFS_REAPURB, for USB FunctionFS.  This is
intended to be used by user space programs, which want to implement a
user space gadget driver.

Only bulk endpoints are currently supported.

Signed-off-by: Ingo Rohloff <ingo.rohloff@lauterbach.com>