usb: renesas_usbhs: gadget: fix NULL pointer dereference in ep_disable()

This patch fixes an issue that the NULL pointer dereference happens
when we uses g_audio driver. Since the g_audio driver will call
usb_ep_disable() in afunc_set_alt() before it calls usb_ep_enable(),
the uep->pipe of renesas usbhs driver will be NULL. So, this patch
adds a condition to avoid the oops.

Signed-off-by: Kazuya Mizuguchi <kazuya.mizuguchi.ks@renesas.com>
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Fixes: 2f98382d (usb: renesas_usbhs: Add Renesas USBHS Gadget)
Cc: <stable@vger.kernel.org> # v3.0+
Signed-off-by: Felipe Balbi <balbi@ti.com>