ethtool: return error from ethnl_ops_begin if dev is NULL

Julian reported that after d43c65b0 Coverity complains about a
missing check whether dev is NULL in ethnl_ops_complete().
There doesn't seem to be any valid case where dev could be NULL when
calling ethnl_ops_begin(), therefore return an error if dev is NULL.

Fixes: d43c65b0 ("ethtool: runtime-resume netdev parent in ethnl_ops_begin")
Reported-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>