iwl3945: iwl3945_mac_get_tsf() should not return zero

The problem fixed here is that iwl3945_mac_get_tsf() returns 0, as the
function is not implemented, and this is considered as a valid value by
the mac layer in mlme.c:1605. The consequence is that the STA in ad-hoc
mode is inserted/removed quite frequently due to IBSS merging.

This patch fixes :
http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1781
and https://bugzilla.redhat.com/show_bug.cgi?id=459401

Signed-off-by: Fabrice Bellet <fabrice@bellet.info>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>