ath6kl: Make foreground scan a deterministic one

This makes the device get into scan state as soon as a
scan request is received, instead of blocking the scan
request till the traffic comes down. It is  necessary
for the deterministic foreground scan particularly when
having multiple vif operating where, if the scan is non-
deterministic, scan on one interface will not start
as long as there are traffic on the other interface. This
change passes 50 msec as foreground scan interval to fix this.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>