ath5k: always extend rx timestamp with tsf

always extend the rx timestamp with the local TSF, since this information is
also needed for proper IBSS merging. this is done in the tasklet for now, maybe
has to be moved to the interrupt handler like in madwifi.

drivers/net/wireless/ath5k/base.c:      Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bruno Randolf <bruno@thinktube.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>