net/mlx5: Unmap only the relevant IO memory mapping

When freeing UAR the driver tries to unmap uar->map and uar->bf_map
which are mutually exclusive thus always unmapping a NULL pointer.
Make sure we only call iounmap() once, for the actual mapping.

Fixes: 0ba42241

 ('net/mlx5: Fix global UAR mapping')
Signed-off-by: Gal Pressman <galp@mellanox.com>
Reported-by: Doron Tsur <doront@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>