staging: ath6kl: Fixing disappearing of scan list due to jiffies wrap over

When jiffies wrap-over, all the BSS in the cache is removed. Wrap-over of
jiffies is not handled in the correct way. This cause the scan list to go
empty during this time for a small duration

Signed-off-by: Vipin Mehta <vmehta@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>