usb: gadget: use config_ep_by_speed() instead of ep_choose()

Remove obsolete functions:
1. ep_choose()
2. usb_find_endpoint()

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>