gadgetfs: get rid of flipping ->f_op in ep_config()

Final methods start with get_ready_ep(), which will fail unless we have
->state == STATE_EP_ENABLED.  So they'd be failing just fine until that
first write() anyway.  Let's do the following:
	* get_ready_ep() gets a new argument - true when called from
ep_write_iter(), false otherwise.
	* make it quiet when it finds STATE_EP_READY (no printk, that is;
the case won't be impossible after that change).
	* when that new argument is true, treat STATE_EP_READY the same
way as STATE_EP_ENABLED (i.e. return zero and do not unlock).
	* in ep_write_iter(), after success of get_ready_ep() turn
	if (!usb_endpoint_dir_in(&epdata->desc)) {
into
	if (epdata->state == STATE_EP_ENABLED &&
	    !usb_endpoint_dir_in(&epdata->desc)) {
- that logics only applies after config.
	* have ep_config() take kernel-side buffer (i.e. use memcpy()
instead of copy_from_user() in there) and in the "let's call ep_io or
ep_aio" (again, in ep_write_iter()) add "... or ep_config() in case it's
not configured yet"

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>