While leaving oper channel, STA informs sleep state to AP to
stop sending data. Till sending ack for the nullfunc, AP
continues to send the data to STA which restarts ps_timer that
is causing unnecessary nullfunc exchange on timer expiry
when the STA was already moved to offchannel. So don't restart ps_timer
on data reception during scan. This issue was identified by
the following warning.

WARNING: at net/mac80211/tx.c:661 invoke_tx_handlers+0xf07/0x1330 [mac80211]
wlan0: Dropped data frame as no usable bitrate found while scanning and
associated. Target station: 00:03:7f:0b:a6:1b on 5 GHz band
Call Trace:
  [<ffffffffa0413ba7>] invoke_tx_handlers+0xf07/0x1330 [mac80211]
  [<ffffffffa0414056>] ieee80211_tx+0x86/0x2c0 [mac80211]
  [<ffffffffa0414345>] ieee80211_xmit+0xb5/0x1d0 [mac80211]
  [<ffffffffa04037e0>] ieee80211_dynamic_ps_enable_work+0x0/0xb0 [mac80211]
  [<ffffffffa04158cf>] ieee80211_tx_skb+0x4f/0x60 [mac80211]
  [<ffffffffa04026e6>] ieee80211_send_nullfunc+0x46/0x60 [mac80211]
  [<ffffffffa0403885>] ieee80211_dynamic_ps_enable_work+0xa5/0xb0 [mac80211]

Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>