[PATCH] ipw2200: wireless extension sensitivity threshold support

The patch allows the user to set the handover threshold, i.e. the number
of consecutively missed beacons that will trigger a roaming attempt. The
disassociation threshold is set to 3 times the handover threshold.

Signed-off-by: Olivier Hochreutiner <olivier.hochreutiner@epfl.ch>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>