mac80211: make listen_interval be limited by low level driver

This patch makes possible for a driver to specify maximal listen interval
The possibility for user to configure listen interval is not implemented
yet, currently the maximum provided by the driver or 1 is used.
Mac80211 uses config handler to set listen interval for to the driver.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>