mac80211: Fix a warning due to skipping tailroom reservation for IV

The devices that require IV generation in software need tailroom
reservation  for ICVs used in TKIP or WEP encryptions.
Currently, decision to skip the tailroom reservation in the tx
path was taken only on whether driver wants MMIC to be generated
in software or not. Following patch appends IV generation check for
such decisions and fixes the following warning.

WARNING: at net/mac80211/wep.c:101 ieee80211_wep_add_iv+0x56/0xf3()
Hardware name: 64756D6
Modules linked in: ath9k ath9k_common ath9k_hw
Pid: 0, comm: swapper Tainted: G        W   2.6.39-rc5-wl
 Call Trace:
[<c102fd29>] warn_slowpath_common+0x65/0x7a
[<c1465c4e>] ? ieee80211_wep_add_iv+0x56/0xf3
[<c102fd4d>] warn_slowpath_null+0xf/0x13
[<c1465c4e>] ieee80211_wep_add_iv+0x56/0xf3
[<c1466007>] ieee80211_crypto_wep_encrypt+0x63/0x88
[<c1478bf3>] ieee80211_tx_h_encrypt+0x2f/0x63
[<c1478cba>] invoke_tx_handlers+0x93/0xe1
[<c1478eda>] ieee80211_tx+0x4b/0x6d
[<c147907c>] ieee80211_xmit+0x180/0x188
[<c147779d>] ? ieee80211_skb_resize+0x95/0xd9
[<c1479edf>] ieee80211_subif_start_xmit+0x64f/0x668
[<c13956fc>] dev_hard_start_xmit+0x368/0x48c
[<c13a8bd6>] sch_direct_xmit+0x4d/0x101
[<c1395ae1>] dev_queue_xmit+0x2c1/0x43f
[<c13a74a2>] ? eth_header+0x1e/0x90
[<c13a7400>] ? eth_type_trans+0x91/0xc2
[<c13a7484>] ? eth_rebuild_header+0x53/0x53
[<c139f079>] neigh_resolve_output+0x223/0x27e
[<c13c6b23>] ip_finish_output2+0x1d4/0x1fe
[<c13c6bc6>] ip_finish_output+0x79/0x7d
[<c13c6cbe>] T.1075+0x43/0x48
[<c13c6e6e>] ip_output+0x75/0x7b
[<c13c4970>] dst_output+0xc/0xe
[<c13c62c9>] ip_local_out+0x17/0x1a
[<c13c67bb>] ip_queue_xmit+0x2aa/0x2f8
[<c138b742>] ? sk_setup_caps+0x21/0x92
[<c13d95ea>] ? __tcp_v4_send_check+0x7e/0xb7
[<c13d5d2e>] tcp_transmit_skb+0x6a1/0x6d7
[<c13d533b>] ? tcp_established_options+0x20/0x8b
[<c13d6f28>] tcp_retransmit_skb+0x43a/0x527
[<c13d8d6d>] tcp_retransmit_timer+0x32e/0x45d
[<c13d8f23>] tcp_write_timer+0x87/0x16c
[<c103a030>] run_timer_softirq+0x156/0x1f9
[<c13d8e9c>] ? tcp_retransmit_timer+0x45d/0x45d
[<c1034d65>] __do_softirq+0x97/0x14a
[<c1034cce>] ? irq_enter+0x4d/0x4d

Cc: Yogesh Powar <yogeshp@marvell.com>
Reported-by: Fabio Rossi <rossi.f@inwind.it>
Tested-by: Fabio Rossi <rossi.f@inwind.it>
Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>