mac80211: mesh: fix potentially unaligned access

The pointer here points directly into the frame, so the
access is potentially unaligned. Use get_unaligned_le16
to avoid that.

Fixes: 3f52b7e3 ("mac80211: mesh power save basics")
Link: https://lore.kernel.org/r/20210920154009.3110ff75be0c.Ib6a2ff9e9cc9bc6fca50fce631ec1ce725cc926b@changeid


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