NET: Proper return code handling in eth_init() function in file eth.c

This patch modifies the return code handling in the eth_init()
function, to be compatible with the handling of the return codes in
the other network stack files. It now returns a 0 on Success and -1 on
error.

Signed-off-by: Upakul Barkakaty <upakul.barkakaty@conexant.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>