enic: do not overwrite error code

In failure path, we overwrite err to what vnic_rq_disable() returns. In
case it returns 0, enic_open() returns success in case of error.

Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Fixes: e8588e26 ("enic: enable rq before updating rq descriptors")
Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>