enic: fix return value in _vnic_dev_cmd

Hardware (in readq(&devcmd->args[0])) returns positive number in case of error.
But _vnic_dev_cmd should return a negative value in case of error.

Signed-off-by: Govindarajulu Varadarajan <_govind@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>