nl80211: fix NL80211_ATTR_CHANNEL_WIDTH attribute type

The new opmode notification used this attribute with a u8, when
it's documented as a u32 and indeed used in userspace as such,
it just happens to work on little-endian systems since userspace
isn't doing any strict size validation, and the u8 goes into the
lower byte. Fix this.

Cc: stable@vger.kernel.org
Fixes: 466b9936

 ("cfg80211: Add support to notify station's opmode change to userspace")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20200325090531.be124f0a11c7.Iedbf4e197a85471ebd729b186d5365c0343bf7a8@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>