ath5k: add notes about rx timestamp

add comments about the fact that we don't know when exactly the atheros
hardware takes the RX timestamp.

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>