cfg80211: make stripping of 802.11 header optional from AMSDU

Currently the devices that have already stripped IEEE 802.11
header from the AMSDU SKB can not use ieee80211_amsdu_to_8023s
routine. This patch enhances ieee80211_amsdu_to_8023s() API by
changing mandatory removing of IEEE 802.11 header from AMSDU
to optional.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>