net/mlx4: Fix fall-through warning 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://github.com/KSPP/linux/issues/115


Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>