net/mlx4: fix initialization of struct members

net/mlx4: 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/mlx4/mlx4_mr.c:357:37: error: missing field 'end'
initializer [-Werror,-Wmissing-field-initializers]
                struct mlx4_mr_cache entry = { 0, };
                                                  ^
../drivers/net/mlx4/mlx4_mr.c:401:36: error: missing field 'end'
initializer [-Werror,-Wmissing-field-initializers]
                        struct mlx4_mr_cache ret = { 0, };
                                                        ^
../drivers/net/mlx4/mlx4_mr.c:691:35: error: missing field 'end'
initializer [-Werror,-Wmissing-field-initializers]
                struct mlx4_mr_cache ret = { 0, };
                                                ^
```

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

Fixes: 9797bfcce1c9 ("net/mlx4: add new memory region support")
Cc: stable@dpdk.org
Signed-off-by: Ali Alnubani <alialnu@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>