usb: dwc3: gadget: always unmap EP0 requests

commit 0416e494 ("usb: dwc3: ep0: correct cache
sync issue in case of ep0_bounced") introduced a bug
where we would leak DMA resources which would cause
us to starve the system of them resulting in failing
DMA transfers.

Fix the bug by making sure that we always unmap EP0
requests since those are *always* mapped.

Fixes: 0416e494

 ("usb: dwc3: ep0: correct cache
	sync issue in case of ep0_bounced")
Cc: <stable@vger.kernel.org>
Tested-by: Tomasz Medrek <tomaszx.medrek@intel.com>
Reported-by: Janusz Dziedzic <januszx.dziedzic@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>