IB/mlx5: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding the new pseudo-keyword fallthrough; instead of
letting the code fall through to the next case.

Link: https://lore.kernel.org/r/2b0c87362bc86f6adfe56a5a6685837b71022bbf.1605896059.git.gustavoars@kernel.org
Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>