Staging: vt6655: Add check dma_mapping_error

This patch checks for dma_mapping_error() after using dma_map_single.
If the dma map fails we release skb buff allocated by dev_alloc_skb() to
avoid possible causes of resource leak.

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>