iwl3945: do not send scan command if channel count zero

Do not send scan command if no channels to scan.

This avoids a Microcode error as reported in:
http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1650
http://bugzilla.kernel.org/show_bug.cgi?id=11806
http://marc.info/?l=linux-wireless&m=122437145211886&w=2

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