net/mlx5: fix environment variable recovery
net/mlx5: fix environment variable recovery

The state of environment variable MLX5_BF_SHUT_UP was not
recovered correctly if there was no tx_db_nc devarg specified.

Fixes: 8409a28573d3 ("net/mlx5: control transmit doorbell register mapping")
Cc: stable@dpdk.org

Signed-off-by: Viacheslav Ovsiienko <5b8db9bb93a9a9aefc8828f79604d1fe963a53dd@mellanox.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk