iwlwifi: W/A for the TSF correction in IBSS

This patch is a W/A for the TSF sync issue in IBSS merging. HW is not
capable to sync TSF (it's constantly little behind). This creates
constant IBSS merging upon reception of each beacon, adding and removing
station which in turn creates above 50% packet loss and thus dramatically
degrade the throughput. The W/A simply stops the driver from declaring it
has a reliable TSF value and thus eliminates IBSS merging.

Signed-off-by: Assaf Krauss <assaf.krauss@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>