regulatory: add NUL to alpha2

alpha2 is defined as 2-chars array, but is used in multiple
places as string (e.g. with nla_put_string calls), which
might leak kernel data.

Solve it by simply adding an extra char for the NULL
terminator, making such operations safe.

Cc: stable@vger.kernel.org
Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>