[MAC80211]: Add LONG_RETRY flag to ieee80211_tx_control

mac80211 informs the driver what the short and long retry values are through
set_retry_limit(), but when packets are being transmitted it did not inform the
driver which of the 2 retry limits should actually be used.
Instead it sends the actual value, but for drivers that can only set the retry limit
and the register and in the descriptor need to indicate which of the limits should
be used this is not really useful.

This patch will add a IEEE80211_TXCTL_LONG_RETRY_LIMIT flag to the
ieee80211_tx_control structure. By default the short retry limit should be
used but if the flag is set the long retry should be used.

This does not prevent the driver to ignore the request for "no retry" packets,
but at least those will be send out with the short retry limit. But there is no
perfect cure for this problem.. :(

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>