IB/mlx5: Remove set-but-not-used variables

Avoid that the compiler complains about set-but-not-used variables when
building with W=1. This patch does not change any functionality.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>