drivers/net: enic: release rtnl_lock on error-path

enic_change_mtu_work() must call rtnl_unlock() on all exiting paths.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Christian Benvenuti <benve@cisco.com>
Cc: Roopa Prabhu <roprabhu@cisco.com>
Cc: Neel Patel <neepatel@cisco.com>
Cc: Nishank Trivedi <nistrive@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>