net/mlx4_en: Remove an unnecessary test

Since this variable is now part of a structure and not allocated dynamically,
this test is irrelevant now.

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>