net/mlx5: allocate device list explicitly

net/mlx5: allocate device list explicitly

At device probing the device list to spawn was allocated
as dynamic size local variable. It was no possible to have
one unified exit point from routine due to compiler warnings.
This patch allocates the spawn device list directly with
rte_zmalloc() and it is possible to goto to unified exit
label from anywhere of the routine.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>