regulatory: add NUL to request alpha2

Similar to the ancient commit a5fe8e76 ("regulatory: add NUL
to alpha2"), add another byte to alpha2 in the request struct so
that when we use nla_put_string(), we don't overrun anything.

Fixes: 73d54c9e

 ("cfg80211: add regulatory netlink multicast group")
Reported-by: Kees Cook <keescook@google.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>