ath9k: Add support for multiple virtual AP interfaces

This patch fixes the TSF offset calculation for staggered Beacon frames
and sets ATH_BCBUF back to the earlier value 4 to enable multi-BSS
configurations of up to four BSSes.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>