libertas: fix mode initialization problem

After moving lbs_find_best_network_ssid() from scan.c to assoc.c gcc was
able to deduce that new_mode might stay uninitialized.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>