mesh: Make inactivity timer configurable

mesh: Make inactivity timer configurable

Current mesh code uses ap_max_inactivity as inactivity timer. This patch
makes it configurable.

There is another mesh inactivity timer in mac80211. The timer works even
if user_mpm=1. So this patch sets the max value to the timer for
workaround.

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>