nl80211: 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 letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/fe5afd456a1244751177e53359d3dd149a63a873.1605896060.git.gustavoars@kernel.org


Signed-off-by: Johannes Berg <johannes.berg@intel.com>