Maintain maximum blacklist count over list clear operations

Maintain maximum blacklist count over list clear operations

wpas_connection_failed() uses the blacklist count to figure out a
suitable time to wait for the next scan. This mechanism did not work
properly in cases where the temporary blacklist gets cleared due to no
other BSSes being available. Address this by maintaining an additional
count of blacklisting values over wpa_blacklist_clear() calls. In
addition, add one more step in the count to timeout mapping to go to 10
second interval if more than four failures are seen.

Signed-hostap: Jouni Malinen <j@w1.fi>