net/mlx4: mlx4_config_dev_retrieval() - Initialize struct config_dev before using

Add Initialization to struct config_dev before filling and using it.
Fix to warning:

warning: config_dev.rx_checksum_val may be used uninitialized in this function

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>