net/mlx5: fix initialization of struct members

net/mlx5: fix initialization of struct members

This patch fixes compilation errors with meson and the clang
compiler caused by some of the struct members not being
initialized.

```
../drivers/net/mlx5/mlx5_mr.c:345:37: error: missing field 'end'
initializer [-Werror,-Wmissing-field-initializers]
                struct mlx5_mr_cache entry = { 0, };
                                                  ^
../drivers/net/mlx5/mlx5_mr.c:389:36: error: missing field 'end'
initializer [-Werror,-Wmissing-field-initializers]
                        struct mlx5_mr_cache ret = { 0, };
                                                        ^
../drivers/net/mlx5/mlx5_mr.c:691:35: error: missing field 'end'
initializer [-Werror,-Wmissing-field-initializers]
                struct mlx5_mr_cache ret = { 0, };
                                                ^
```

The compilation errors reproduce with
clang version 3.4.2 (tags/RELEASE_34/dot2-final) on RHEL.

Fixes: e1114ff6a5ab ("net/mlx5: support e-switch flow count action")
Fixes: db48f9db5d9f ("net/mlx5: support new flow counter API")
Fixes: 974f1e7ef146 ("net/mlx5: add new memory region support")
Fixes: 65c9d24170c9 ("net/mlx5: enable loopback by configured mode")
Fixes: 87011737b715 ("mlx5: add software counters")
Cc: stable@dpdk.org
Signed-off-by: Ali Alnubani <alialnu@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>