udc: core: Add handling for case where allocated buffer is not mapped

In BAM2BAM case, USB request is allocated but it is used as dummy
buffer. Hence it has never been map using DMA API. There is crash
seen with USB composition switch if this allocated but not mapped
buffer is being unmapped. Fix this issue by explicitly adding check
DMA_ERROR_CODE.

Change-Id: I0e9e9f4ea1a600ab6de1e1e7b23e797d89d32bab
Signed-off-by: Mayank Rana <mrana@codeaurora.org>