mac80211: remove one user of ieee80211_get_hdr_info

ccmp_special_blocks was only using it to calculate data_len,
calculate that directly.

Use unaligned helpers rather than masking/shifting.

Use symbolic constants for the masked frame_control, and do it directly
on a le16 value.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>