ethoc: use devres resource management

The point of using the devres resource management routines is that they
simplify the driver by taking care of releasing resources on failure and
release.  A recent commit added a bunch of error handling that is unnecessary
in this context.

This patch removes this redundant error handling, as well as using
dmam_alloc_coherent in place of dma_alloc_coherent in order to use this
framework consistenly throughout the driver.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: David S. Miller <davem@davemloft.net>