mac80211: clarify alignment docs, fix up alignment

Not all drivers are capable of passing properly aligned frames,
in particular with mesh networking no hardware will support
completely aligning it correctly.

This patch adds code to align the data payload to a 4-byte
boundary in memory for those platforms that require this, or
when CONFIG_MAC80211_DEBUG_PACKET_ALIGNMENT is set.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>