ath6kl: don't force foreground scan when connected

In my setup data transfer stalls when there's data transmission during
scan. After some testing I found out that using background scan
when connected to makes the problem go away. This is more like
a workaround than a proper fix, but as the stall is so severe the
workaround is justified.

With a dual band card this increases scan time when connected from
1.9s to 4.4s. When not connected the scan time is not affected and
is the same 1.9s.

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