usb: renesas_usbhs: gadget: remove usbhsg_uep_init()

Current driver always initialized uep->pipe to NULL on usbhsg_try_start().
But it breaks relationship with
usb_ep_ops :: enable/disable functions when suspend/resume.
This patch solved this issue by initializing uep->pipe on probe()

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>