wl1271: make sure PS is disabled in PLT

We cannot be in PS mode when running PLT tests, so we need to make sure we're
in active mode.  Also, we need to clear up the rx_counter when we stop PLT,
otherwise it could cause problems when entering PLT again.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>