ixgb: Add missing dma_mapping_error-call in ixgb_alloc_rx_buffers

After dma_map_single, dma_mapping_error must be called. It seems safe to
not free the skb allocated in this function, as the skb can be reused
later.

Additionally this patch fixes one coding-style error.

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>