nl80211: correctly check NL80211_ATTR_REG_ALPHA2 size

We need this to be at least two bytes, so we can access
alpha2[0] and alpha2[1]. It may be three in case some
userspace used NUL-termination since it was NLA_STRING
(and we also push it out with NUL-termination).

Cc: stable@vger.kernel.org
Reported-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220411114201.fd4a31f06541.Ie7ff4be2cf348d8cc28ed0d626fc54becf7ea799@changeid


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