mac80211: shorten the passive dwell time for sw scans

mac80211's software scan implementation uses a passive dwell time of
(HZ / 5) which means we stay 200ms on each passive channel. Compared
to iwlwifi's hw scan and the old ipw* drivers which use values around
120ms this is quite long.

Reducing the passive dwell time from 200ms to 125ms should save us
something around a second on cards capable of 11a and we should still be
able to catch beacons from most access points (assuming a ~100ms beacon
interval).

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>