iwlwifi: clean key table in iwl_clear_stations_table function

This patch cleans uCode key table bit map iwl_clear_stations_table
since all stations are cleared also the key table must be.

Since the keys are not removed properly on suspend by mac80211
this may result in exhausting key table on resume leading
to memory corruption during removal

This patch also fixes a memory corruption problem reported in
http://marc.info/?l=linux-wireless&m=122641417231586&w=2 and tracked in
http://bugzilla.kernel.org/show_bug.cgi?id=12040.

When the key is removed a second time the offset is set to 255 - this
index is not valid for the ucode_key_table and corrupts the eeprom pointer
(which is 255 bits from ucode_key_table).

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Reported-by: Carlos R. Mafra <crmafra2@gmail.com>
Reported-by: Lukas Hejtmanek <xhejtman@ics.muni.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>