net: xilinx_emacps: Add dma_mapping_error checking

CONFIG_DMA_API_DEBUG enabled kernel showed backtraces indicating that
"DMA-API: device driver failed to check map error [device
address=0x000000002e997802] [size=322 bytes] [mapped as single]"

Add the check, unwind the mappings if it is a fragmented frame and if the
check is failed then free the skb

Reported-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>