usb: ci13xxx_udc: unmap req DMA buffers upon cable disconnection

Device controller driver transfers the ownership of usb request buffer
memory to device DMA by making call to dma_map_single. CPU regains the
access only after making call to dma_unmap_single.
Upon request completion USB driver is performing dma_unmap_single and
accessing the memory.
Perform dma_unmap_single upon cable disconnect also while flushing the USB
requests.

Change-Id: Ic180c2b1158c93c5b187607c364f0fd5056cc52f
CRs-fixed: 327466
Signed-off-by: Anji jonnala <anjir@codeaurora.org>