ath9k: fix beaconing for mesh interfaces

Mesh beaconing on ath9k was broken by this commit:

commit 4801416c
Author: Ben Greear <greearb@candelatech.com>
Date:   Sat Jan 15 19:13:48 2011 +0000

This patch assigns the right opmode when the device is used in mesh
mode.

Reported-by: Fabrice Deyber <fabricedeyber@agilemesh.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>