cfg80211: limit wiphy names to 128 bytes

There's currently no limit on wiphy names, other than netlink
message size and memory limitations, but that causes issues when,
for example, the wiphy name is used in a uevent, e.g. in rfkill
where we use the same name for the rfkill instance, and then the
buffer there is "only" 2k for the environment variables.

This was reported by syzkaller, which used a 4k name.

Limit the name to something reasonable, I randomly picked 128.

Reported-by:  <syzbot+230d9e642a85d3fec29c@syzkaller.appspotmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>