usb: gadget: udc: renesas_usb3: Fix return value of usb3_write_pipe()

This patch fixes an issue that this driver cannot go status stage
in control read when the req.zero is set to 1 and the len in
usb3_write_pipe() is set to 0. Otherwise, if we use g_ncm driver,
usb enumeration takes long time (5 seconds or more).

Fixes: 746bfe63 ("usb: gadget: renesas_usb3: add support for Renesas USB3.0 peripheral controller")
Cc: <stable@vger.kernel.org> # v4.5+
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>