wl1271: Rewrite hardware keep-alive handling

The driver had a join command without keep-alive restart procedures in the
channel changing code. After associated scans, the mac80211 does re-set the
current channel, causing the join to occur. This would stop the hardware
keep alive.

To make the joins safer in this respect, this patch adds a join function that
does the hardware-keep-alive magic along the join. This is now invoked in the
above mentioned scenario, and also other scenarios.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>