wireless: correct sparse warning in lib80211_crypt_tkip.c

  CHECK   net/wireless/lib80211_crypt_tkip.c
net/wireless/lib80211_crypt_tkip.c:581:27: warning: cast to restricted __le16

Caused by dereferencing a "u8 *" and passing it to le16_to_cpu...

Signed-off-by: John W. Linville <linville@tuxdriver.com>