Revert "usb: gadget: Store endpoint pointer in usb_request"

This reverts commit 167ac21a which is
commit bfb1d99d upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I4ff93664eb185b1a6a6019b457e41ef7e7ff35c9
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>