IB/mlx5: fix debugfs cleanup

If delay_drop_debugfs_init() fails in any of the operations to create
debugfs, it is calling delay_drop_debugfs_cleanup() as part of its
cleanup. But delay_drop_debugfs_cleanup() checks for 'dbg' and since
we have not yet pointed 'dbg' to the debugfs we need to cleanup, the
cleanup fails and we are left with stray debugfs elements and also a
memory leak.

Fixes: 4a5fd5d2 ("IB/mlx5: Add necessary delay drop assignment")
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>