ath5k: fix mesh beaconing

This patch is from Nick Kossifidis but he forgot to send it.  It ensures
that the beacon queue gets started in mesh mode as well, otherwise ath5k
will not beacon in mesh point mode.

At this time, we still need to issue a scan before mesh beaconing will
work but that appears to be a separate problem.

Signed-off-by: Andrey Yurovsky <andrey@cozybit.com>
Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>