e100: Release skb when DMA mapping is failed in e100_xmit_prepare

When pci_dma_mapping_error in e100_xmit_prepare is failed, the skb buffer
allocated by netdev_alloc_skb_ip_align in e100_rx_alloc_skb is not
released, which causes a possible resource leak.
This patch adds error handling code to fix it.

Signed-off-by: Jia-Ju Bai <baijiaju1990@163.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>