mac80211: rename max_sleep_interval to max_sleep_period

Kalle points out that max_sleep_interval is somewhat confusing
because the value is measured in beacon intervals, and not in
TU. Rename it to max_sleep_period to be consistent with things
like DTIM period that are also measured in beacon intervals.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>