wl1251: add missing __packed modifier for some structs

Several acx and cmd structures are missing __packed modifier, add it.
This was noticed while comparing them with corresponding wl1271 code.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>