net/mlx4: fix minor resource leak during init

net/mlx4: fix minor resource leak during init

Temporary IB device context and list are not freed in case of a successful
initialization of the device.

This issue is caused by the two following commits, the first of which
causes initialization to return early, while the second one goes a bit
overboard while switching to negative errno values; an internal variable
(err) is needed to tell success from failure at the end of the function
since rte_errno is not reliable enough.

Fixes: f2318196c71a ("net/mlx4: remove limitation on number of instances")
Fixes: 9d14b27308a0 ("net/mlx4: standardize on negative errno values")
Cc: stable@dpdk.org
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>