wl1271: Fix keep-alive related command error

The firmware does not like the host configuring the keep-alive after it has
been once configured after a join-operation. Instead, it will complain about
invalid parameters, which do not break functionality, but do not look nice
in the syslog either.

This patch prevents the complaints by only configuring the keep-alive once
for an association, after the first time join is performed with the correct
bssid.

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>