drivers/ethernet: Fix and clean a bit ENC28J60 driver

Driver is still a bit messy, not using for instance the 0-copy
capabilities of SPI API, but this will be fix later.

Constify most of the spi buf structures, removing useless variables,
renaming function with common prefix eth_enc28j60_ etc...

It seems to fix an issue on verifying if tx was successful as well.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>