wifi: Only refresh location on BSS list changes

If BSS list hasn't changed since the last location refresh, not much point
in refreshing location and sending redundant query to MLS.

Fixes #100.
3 jobs for master in 3 minutes and 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #72386
ubuntu-16.04

00:02:09

passed #72387
ubuntu-17.10

00:03:05

passed #72388
ubuntu-18.04

00:02:47