usb: renesas_usbhs: gadget: fix re-enabling pipe without re-connecting

This patch fixes an issue that the renesas_usbhs driver in gadget mode
cannot work correctly even if I disabled DMAC of the driver when I used
the g_zero driver and the testusb tool.

When a usb cable is re-connected, the renesas_usbhs driver calls the
usbhsp_flags_init() (via usbhs_hotplug() --> usbhs_mod_call(start) -->
usbhsg_try_start() --> usbhs_pipe_init()). However, the driver doesn't
call the usbhsp_flags_init() when usbhsg_ep_disable() is called.
So, if a gadget driver calls usb_ep_enable() and usb_ep_disable() again
and again, the renesas_usbhs driver will output the following log:

  renesas_usbhs renesas_usbhs: can't get pipe (BULK)
  renesas_usbhs renesas_usbhs: wrong recip request

Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>