rtl818x_pci: Fix a memory leak in rtl8180_init_rx_ring

When dev_alloc_skb or pci_dma_mapping_error in rtl8180_init_rx_ring fails,
the memory allocated by pci_zalloc_consistent is not freed.

This patch fixes the bug by adding pci_free_consistent
in error handling code.

Signed-off-by: Jia-Ju Bai <baijiaju1990@163.com>
Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>