[PATCH] ipw2200: use generic ieee80211_get_hdrlen() to get packet length

replace ipw2200 specific frame_hdr_len() with generic
ieee80211 routine ieee80211_get_hdrlen()

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>