staging: emxx_udc: Add checks for dma_alloc_coherent()

As the dma_alloc_coherent may return NULL, the return value needs to be
checked to avoid NULL poineter dereference.

Signed-off-by: Yuan Can <yuancan@huawei.com>
Reviewed-by: Simon Horman <horms@verge.net.au>
Link: https://lore.kernel.org/r/20230119083119.16956-1-yuancan@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>