wcn36xx: Don't run scan_init multiple times

Run scan_init only once. There's no need to run this command multiple times
if it has already been run once.

The software scan algorithm can end up repeatedly calling scan_init on each
loop resulting in between four and eight milliseconds of lost time on each
callout.

Subtract the overhead now.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>