iwlegacy: add flush callback

Dump implementation of flush, which just wait until all TX queues
become empty.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

diff --cc drivers/net/wireless/iwlegacy/3945-mac.c
Simple merge

diff --cc drivers/net/wireless/iwlegacy/4965-mac.c
Simple merge

diff --cc drivers/net/wireless/iwlegacy/common.c
Simple merge

diff --cc drivers/net/wireless/iwlegacy/common.h
Simple merge