mac80211: change number of pre-assoc scans

This patch fixes noticed problem in noisy environments of 50+ APs
that scan fails to find the requested AP on first try, which
leads to connection refusal. second scan has empirically proven to fix
this problem in almost all cases.

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Esti Kummer <ester.kummer@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>