iwl3945: avoid redundant iwl3945_get_active_dwell_time

iwl3945_get_active_dwell_time is called twice within the same context.
The patch removes one instance.

Signed-off-by: Abhijeet Kolekar <abhijeet.kolekar@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>