Set transmit power on config()

This alleviates any potential issue with transmit power settings
surviving from aborted tests.  It also affords the option to set
txpower as a parameter to "config".

BUG=chromium-os:11591
TEST=Reran tests

Change-Id: I32d62ed286913e6df06e09781234c0f3c9956481

Review URL: http://codereview.chromium.org/6250109
1 file changed