mlx5: Remove a set-but-not-used variable

This has been detected by building the mlx5 driver with W=1.

Fixes: 1a412fb1 ('net/mlx5: Fixes: 1a412fb1 (IB/mlx5: Modify QP
commands via mlx5 ifc')
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Eli Cohen <eli@mellanox.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>