[PATCH] libertas: scan two channels per scan command

Scan two channels per each command on set_scan(), then bail out and let
get_scan() continue the scanning work up to the last channel.

This gives time to the firmware so it can go back to the association
channel and keep the connection alive.

Fixes http://dev.laptop.org/ticket/841

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>