iwlwifi: better station table maintenance

This patch makes the station table maintenance safer. Two flags are
maintained:

1) if station is present in driver
2) if station is present in uCode

This will allow us in the future to deal with more stations than the
firmware allows.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@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>