mac80211: hardware scan rework

The scan code in mac80211 makes the software scan assumption in various
places. For example, we stop the Tx queue during a software scan so that
all the Tx packets will be queued by the stack. We also drop frames not
related to scan in the software scan process. But these are not true for
hardware scan.

Some wireless hardwares (for example iwl3945/4965) has the ability to
perform the whole scan process by hardware and/or firmware. The hardware
scan is relative powerful in that it tries to maintain normal network
traffic while doing a scan in the background. Some drivers (i.e iwlwifi)
do provide a way to tune the hardware scan parameters (for example if the
STA is associated, what's the max time could the STA leave from the
associated channel, how long the scans get suspended after returning to
the service channel, etc). But basically this is transparent to the
stack. mac80211 should not stop Tx queues or drop Rx packets during a
hardware scan.

This patch resolves the above problem by spliting the current scan
indicator local->sta_scanning into local->sta_sw_scanning and
local->sta_hw_scanning. It then changes the scan related code to be aware
of hardware scan or software scan in various places. With this patch,
iwlwifi performs much better in the scan-while-associated condition and
disable_hw_scan=1 should never be required.

Cc: Mohamed Abbas <mohamed.abbas@intel.com>
Cc: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>