net/mlx4_core: Fix uninitialized variable compilation warning

Initialize the uid variable to zero to avoid the compilation warning.

Fixes: 7a89399f ("net/mlx4: Add mlx4_bitmap zone allocator")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>