Fragment number field (part of i_seq) is used for AAD calculation;
as a result, without this patch every driver without h/w crypto support
need to clear it before ieee80211_crypto_encap().
Also fixes rtwn(4) A-MPDU Tx with dev.rtwn.%d.hwcrypto tunable
set to 0 (h/w crypto is disabled).
Tested with:
* Intel 6205, STA mode.
* RTL8188EU, STA mode.
Differential Revision: https://reviews.freebsd.org/D10753