wl1271: wait for join command complete event

Poll for join command completion instead of waiting blindly for 10 msecs.
There is a timeout of 100 msecs, if the command doesn't complete by then, we
return an error code.

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>