iwl3945: improve 3945 leds

'tpt' is a delta throughput (number of packets) and is corelated
to brightness of the LED. We already maintain a delta of packets in
rxtxpackets. There is no need to calculate this delta again which
was affecting the behaviour of LEDS.

Also add two new callback functions for ASSOCIATED/DISASSOCIATED states
where LED's will be *on* for associated state and *off* for disassociated state.

This fixes
http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1771.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>