ath5k: fix mesh point operation

This patch fixes mesh point operation (thanks to YanBo for pointing
out the problem): make mesh point interfaces start beaconing when
they come up and configure the RX filter in mesh mode so that mesh
beacons and action frames are received.  Add mesh point to the check
in ath5k_add_interface.  Tested with multiple AR5211 cards.

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