net/ethoc: fix null dereference on error exit path

priv is assigned to NULL however some of the early error exit paths to
label 'free' dereference priv, causing a null pointer dereference.

Move the label 'free' to just the free_netdev statement, and add a new
exit path 'free2' for the error cases were clk_disable_unprepare needs
calling before the final free.

Fixes issue found by CoverityScan, CID#113260

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>