Not setting the opmode properly during initialization
results in the firmware sending up a bunch of packets
before add_interface() has been called, for the first
interface.

This patch fixes the issue by setting the initial mode
to 'managed'.
Signed-off-by: NSujith <Sujith.Manoharan@atheros.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>