lib80211: Introduce TKIP_HDR_LEN define for code clarity

Introduce TKIP_HDR_LEN define for code clarity (in the same way as
CCMP_HDR_LEN).

Also odd len variable (not used) dropped from lib80211_tkip_hdr().

Signed-off-by: Andriy V. Tkachuk <andrit@ukr.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>