IB/mlx4: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of just letting the code
fall through to the next case.

Link: https://lore.kernel.org/r/0153716933e01608d46155941c447d011c59c1e4.1605896059.git.gustavoars@kernel.org
Link: https://github.com/KSPP/linux/issues/115


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