usb: gadget: udc: core: don't starve DMA resources

Always unmap all SG entries as required by DMA API

Fixes: a698908d ("usb: gadget: add generic map/unmap request utilities")
Cc: <stable@vger.kernel.org> # v3.4+
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>