net/mlx5: Fix command bad flow on command entry allocation failure

When driver fail to allocate an entry to send command to FW, it must
notify the calling function and release the memory allocated for
this command.

Fixes: e126ba97 ('mlx5: Add driver for Mellanox Connect-IB adapters')
Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Cc: kernel-team@fb.com
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>