mac80211: give burst time in txop rather than 0.1msec units

This changes mac80211 to pass the burst time to conf_tx in txop
units rather than 0.1msec units. 0.1msec units are only required
by atheros hardware (according to current driver support), all
other drivers do other calculations or require the txop value.
Therefore, it results in fewer calculations and more precision
if we just pass the txop value through to the driver.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>