stmmac: fix init_dma_desc_rings() to handle errors

In stmmac_init_rx_buffers():
* add missing handling of dma_map_single() error
* remove superfluous unlikely() optimization while at it

Add stmmac_free_rx_buffers() helper and use it in dma_free_rx_skbufs().

In init_dma_desc_rings():
* add missing handling of kmalloc_array() errors
* fix handling of dma_alloc_coherent() and stmmac_init_rx_buffers() errors
* make function return an error value on error and 0 on success

In stmmac_open():
* add handling of init_dma_desc_rings() return value

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>