enic: Use dma_set_mask_and_coherent()

Use dma_set_mask_and_coherent() instead of unrolling it with some
dma_set_mask()+dma_set_coherent_mask().

This simplifies code and removes some dead code (dma_set_coherent_mask()
can not fail after a successful dma_set_mask())

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>