mlx4_core: Move handling of MLX4_QP_ST_MLX to proper switch statement

The handling of MLX4_QP_ST_MLX in verify_qp_parameters() was
accidentally put inside the inner switch statement (that handles which
transition of RC/UC/XRC QPs is happening).  Fix this by moving the case
to the outer switch statement.

The compiler pointed this out with:

    drivers/net/ethernet/mellanox/mlx4/resource_tracker.c: In function 'verify_qp_parameters':
 >> drivers/net/ethernet/mellanox/mlx4/resource_tracker.c:2875:3: warning: case value '7' not in enumerated type 'enum qp_transition' [-Wswitch]
       case MLX4_QP_ST_MLX:

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 99ec41d0 ("mlx4: Add infrastructure for selecting VFs to enable QP0 via MLX proxy QPs")
Signed-off-by: Roland Dreier <roland@purestorage.com>