net/mlx4: suppress 'may be used uninitialized' warning

gcc 4.8.4 complains that mlx4_SW2HW_MPT_wrapper() uses an uninitialized
'mpt' variable:
  drivers/net/ethernet/mellanox/mlx4/resource_tracker.c: In function 'mlx4_SW2HW_MPT_wrapper':
  drivers/net/ethernet/mellanox/mlx4/resource_tracker.c:2802:12: warning: 'mpt' may be used uninitialized in this function [-Wmaybe-uninitialized]
     mpt->mtt = mtt;

I think this warning is a false complaint.  mpt is only used when
mr_res_start_move_to() return zero, and in all such cases it initializes
mpt.  But apparently gcc cannot see that.

Initialize mpt to avoid the warning.

Signed-off-by: Greg Thelen <gthelen@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>