ath9k_htc: Initialize HW opmode

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: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>