mac80211: Add attribute aligned(2) to struct 'action'

During refactor in commit 9e478066 ("mac80211: fix MU-MIMO
follow-MAC mode") a new struct 'action' was declared with packed
attribute as:

  struct {
          struct ieee80211_hdr_3addr hdr;
          u8 category;
          u8 action_code;
  } __packed action;

But since struct 'ieee80211_hdr_3addr' is declared with an aligned
keyword as:

  struct ieee80211_hdr {
  	__le16 frame_control;
  	__le16 duration_id;
  	u8 addr1[ETH_ALEN];
  	u8 addr2[ETH_ALEN];
  	u8 addr3[ETH_ALEN];
  	__le16 seq_ctrl;
  	u8 addr4[ETH_ALEN];
  } __packed __aligned(2);

Solve the ambiguity of placing aligned structure in a packed one by
adding the aligned(2) attribute to struct 'action'.

This removes the following warning (W=1):

  net/mac80211/rx.c:234:2: warning: alignment 1 of 'struct <anonymous>' is less than 2 [-Wpacked-not-aligned]

Cc: Johannes Berg <johannes.berg@intel.com>
Suggested-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>