net/mlx4_core: Fix several coding style errors

Fix 3 checkpatch errors within mlx4/main.c:
- Unnecessary mlx4_debug_level global variable initialization to 0.
- Prohibited space before comma.
- Whitespaces instead of tab.

Signed-off-by: Erez Alfasi <ereza@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>