wait for sched_scan completion before starting a new one

trying to start a sched scan before getting the previous
sched scan's completion results in some out-of-sync state
between the supplicant and the driver.

Signed-off-by: Eliad Peller <eliad@wizery.com>
smart_config: scan social channels first

scan on channels 1/6/11 first (that is the current requirement.
it doesn't seem very useful, though...)

we can also simplify the code by letting the driver check
whether the channel is disabled. however, the rest of the
scan code checks it in userspace as well, so do it also
here.

Signed-off-by: Eliad Peller <eliad@wizery.com>