network_WiFi_CSADisconnect: alternate AP channel for each test iteration

Some drivers (marvell 8897) disallow client from reconnecting back to the
same AP on the same channel immediately after the AP send out a CSA
to switch to a different channel. Work around this problem by alternating
AP channel for each test iteration.

BUG=chrome-os-partner:33338
TEST=run this test with Marvell 8897 device (nyan_big).

Change-Id: I9699c118afb7782b999bd99f0f3f4a7f0d0a308a
Reviewed-on: https://chromium-review.googlesource.com/227166
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>
2 files changed